Doreen Foy Organic Chemistry Laboratory 1 The Synthesis of Alkenes ...
Doreen Foy Organic Chemistry Laboratory 1 The Synthesis of Alkenes: The Dehydration of Cyclohexanol Hannah Loch 11/11/2012
Abstract The goal of this experiment was to synthesize cyclohexene from cyclohexanol by unimolecular elimination (E1) through the dehydration of cyclohexanol (scheme1) Phosphoric acid was used to catalyze the reaction by providing the proton necessary for the conversion of the bad OH- group to water which is a good leaving group. The unimolecular elimination was favored over unimolecular substitution by heating the reaction at a high temperature and also by the use of the non-nucleophilic phosphoric acid. A bromine test and Infrared spectroscopy was done to test the presence of an alkene in the product formed by dehydration which indeed confirmed the presence of an alkene. The mass of cyclohexene collected from the dehydration was 4.73g and the percent yield of cyclohexene was 58.51%.
Introduction Alkenes are hydrocarbons which have carbon–carbon double bonds (σ bond and π bond) and are one of the many functional groups in organic molecules. Alkenes are sp2 hybridized and are unsaturated because they are missing two hydrogens from the saturated alkane formula (CnH2n+2). Typically alkenes are synthesized by elimination reactions. Apart from elimination reactions, organic molecules can also undergo substitution reactions amongst many other reactions like oxidation and reduction based on the structure of the organic molecule and the conditions under which the reaction is performed. All of these reactions are possible because the carbon atom in organic molecules is electron deficient due to induced dipoles created by the presence of functional groups or more electronegative atoms/groups. The organic molecule as a
whole is referred to as the substrate. The carbon atom in the organic molecule which is of interest to be attacked is referred to as the electrophile and it is usually bonded to a more electronegative atom/group such as a halide, oxygen or psuedohalide, called the leaving group. In a reaction, a leaving group leaves the substrate by heterolysis (a reaction in which the breaking of bonds leads to the formation of ion pairs) and it is replaced by a nucleophile (usually a base) which is attracted to the partial positive carbon atom because the nucleophile has excess electron pairs or a negative charge. The relative strength of a nucleophile determines its nucleophilicity. Nucleophilicity depends on many factors, including charge, basicity, solvent, polarizability, and the nature of the substituents present on the organic molecule. In general, a nucleophile containing a negatively charged reactive atom is better than a nucleophile containing a reactive atom that is neutral and basicity parallels nucleophilicity meaning a strong base is a good nucleophile and a weak base is a weak nucleophile. Also, there are several factors that contribute to the ability of a group/atom to function as a good leaving group which includes the property of the carbon-leaving group bond (polarizability and strength), and the stability of the leaving group. Weak bases are good leaving groups and strong bases are bad leaving groups. The substrate organic molecule containing the electrophile is characterized in terms of steric effect which entails the bulkiness of the substrate and its shape/size in 3D and whether the electrophilic carbon is a primary, secondary or tertiary carbon. The nature of the substrate, nucleophile, electrophile and solvent (polar protic or polar aprotic) used will determine whether an organic molecule will undergo a substitution or elimination reaction. An elimination reaction is one in which an α- hydrogen and a leaving group are removed from an organic molecule in either a one step or a two-step mechanism to form an alkene. The
one and two-step mechanisms are bimolecular (E2) and unimolecular (E1) elimination reactions respectively, defined based on the rates of the reactions. The rates of the reactions are based on the kinematics of each reaction and not on the number of steps in the reaction. As implied by the names, E2 reactions have a rate factor of two (second-order) because the rate of the reaction is based on the concentration of the substrate and the nucleophile while E1 reactions have a rate factor of one (first-order) because the rate of the reaction is based solely on the concentration of the substrate. E2 is a one-step concerted process with a single transition state typically undergone by primary and secondary substrates (scheme2) while E1 is a two-step process of elimination (carbocation formation and deprotonation) undergone by secondary and tertiary substrates only (scheme3). E2 requires a reasonably good nucleophile (strong base) and a polar protic solvent. E1 is favored by tertiary and secondary substrates structures with a bad nucleophile in the presence of a polar protic solvent in which the tertiary substrate is more reactive than the secondary due to greater stabilization of the carbocation and lower activation energy. The polar protic solvent also helps stabilize the carbocation. In an E1 reaction, the most substituted alkene (most stable) is preferred over the least substituted one according to Saytaev’s rule (scheme4). This is due to the stabilization of the carbocation intermediate formed by hyperconjugation which leads to rearrangements involving hydride shifts or alkyl shifts in which a hydrogen or an alkyl group with its pair of electrons relocate to the carbocation to form a more stable carbocation.
A substitution reaction is one in which a leaving group on a substrate is replaced by a nucleophile in either a one step or two step processes, which is a bimolecular substitution (Sn2) and unimolecular substitution (Sn1) respectively. Sn2 is undergone by primary and secondary substrates only preferable in a polar aprotic solvent with a relatively good nucleophile while Sn1 is undergone by secondary and tertiary substrates only preferably in a polar protic solvent with a poor nucleophile. Tertiary substrates are better than secondary substrates for Sn1 also because of the stabilization of the carbocation with the polar protic solvent increasing the stability of the carbocation. There is always competition between substitution and elimination reactions depending of the conditions of the reaction. Under normal conditions (room temperature, relative nucleophile, polar protic solvent etc.), Sn1 is always accompanied by E1 as the minor product but E1 products can be favored by changing the conditions of the reaction such as higher temperatures which favor E1 products. A common way to synthesize alkenes is by the dehydration of alcohols (scheme5). The reaction is acid catalyzed to transform the hydroxide (OH-) bad leaving group into a good leaving group (H2O) and so a basic nucleophile cannot be used because it cannot provide the proton needed to transform OH- to H2O. The acid used has to be non-nucleophilic such as phosphoric acid or sulfuric acid in a high concentration for an E1 product to be favored over a Sn1 product. High temperatures will also favor E1 over Sn1 due to the energetics of the reaction
as Gibbs free energy (ΔG) becomes significantly negative. The entropy of a Sn1 reaction is zero while the entropy of an E1 reaction is greater than zero, so increasing the temperature make ΔG even more negative increasing the spontaneity of E1 (scheme6).
To test the effectiveness of a reaction producing an alkene, the product of the reaction can be tested for the presence of an alkene using the bromine test or infrared spectroscopy. The bromine test occurs by halogenation at the double bond in which two bromine radicals break the double bond by binding to the two carbons in the double bond. The reaction occurs as a bromine radical attacks one of the sides of the p-orbital in a pi-bond forcing the bond to cleave homolytically and a bromine-carbon bond is formed at the point where the p-orbital of the carbon was attacked. The presence of an alkene is indicated by the disappearance of the deep brown coloration of bromine, which happens because the bromine has been consumed by the reaction of the unknown sample. Infrared spectroscopy (IR) is a technique by which a molecule is analyzed based on its absorbance of infrared light due to the functional groups present within the molecule in a single non-destructive test. IR is possible because every bond has a natural frequency and the amount of energy applied to a bond affects the amplitude of the vibration of a bond. Therefore it is possible to detect functional groups on an IR spectrum, as each functional group will have a specific natural frequency that is characteristic of that group, which corresponds to a specific absorption region of the spectrum. The natural vibration frequency of
any bond relies on its bond order (single, double, triple) and the bonded atoms such as hydrogen, oxygen or nitrogen as determined by Hooke’s law (scheme7). Hooke’s law gives a basic idea of how the bond between atoms will act, in comparison to a spring, with the two atoms acting like weights attached to the spring. By performing an IR spectroscopy on the product of a reaction, it is possible to discern whether a reaction took place, or if the right reaction took place, by analyzing the IR spectrum for that product, comparing the peaks to standard peaks. Alkenyl alkenes have a standard absorption frequency of 1620cm-1-1680cm-1 and aromatic alkenes have a standard absorption frequency of 1500cm-1&1600cm-1 and so these peaks (aromatic or alkenyl)1 have to be present in the IR spectrum of the product to indicate the presence of an alkene.
Reagent table 1&2 Name,
Molecular
Structure
weight(g/mol) used
Cyclohexanol 100.16
Cyclohexene
84.16
Amount Concentration Density(g/ml) Melting Boiling point
point
(0C)
(0C)
10mL
0.962
25.93
160.84
4.73g
0.811
-103.5
82.98
98.00
12mL
Bromine
79.90
Anhydrous
Phosphoric
85%
1.88
42.35
158
≈3drops
3.103
-7.2
58.8
142.04
≈2g
2.68
884
1429
58.44
20mL
2.16
801
1413
acid
NaSO4
NaCl
Experimental An amount of 10mL cyclohexanol, 12mL of 85% phosphoric acid and 3 charcoal boiling chips were added to a 100mL round bottom flask. A simple distillation apparatus was assembled with a 50mL round bottom flask immersed in an ice-water bath used as the receiving flask and the heating mantle was set to 55%. The temperature at which the distillate collected was recorded to be 75.50C . The distillate (cyclohexene) was transferred to a separatory funnel and 5mL of 10% Na2CO3 was added to it. The mixture was washed twice with 2 portions of 10mL saturated NaCl with the cyclohexene layer collected. The cyclohexene was placed in a 25ml Erlenmeyer flask and dry anhydrous NaSO4 was added to it to dry the product. The dried cyclohexene product was placed in a pre-weighed round bottom flask & beaker (167.82g) and weighed to be 172.55g. The mass of the cyclohexene produced was determined to be 4.73g and the percent
yield of cyclohexene was calculated to be 58.51%. A bromine test and IR spectroscopy was done for the product to test the presence of an alkene. Results Temperature at which stable distillate was collected=75.50C Mass of cyclohexene collected= Mass of beaker, round flask and product - mass of beaker and empty round bottom flask =172.55g-167.83g= 4.73g % Yield cyclohexene= Moles of cyclohexene X 100 Moles of cyclohexanol Moles of cyclohexanol=
Moles of cyclohexene=
% Yield of cyclohexene=
IR results Wavenumber (1/cm)
Bond
1437.65-639.95
fingerprint region
1652.26
Akenyl(C=C)
2837.41
C (sp3) –H
2928.23
C(sp3)-H
2983.45
C(sp3)-H
3022.12
C (sp2) –H
3061.69
C (sp2) –H
Cyclohexene is an oily clear liquid and when bromine was added to it, the color changed to deep brown. As the reaction proceeded, the deep brown color disappeared until the mixture became clear again. Discussion An alkene (cyclohexene) is synthesized in this experiment by the dehydration of an alcohol (cyclohexanol) through the loss of water to form a double bond (Scheme1). The dehydration proceeded by the heating of cyclohexanol in the presence of phosphoric acid at a high temperature. Phosphoric acid was used because it is a strong acid and so it can dissociate completely to form ions thereby providing the proton (H+) needed for the conversion of the bad OH- leaving group to water which is a good leaving group. The OH- group in the alcohol donates two electrons to H+ from the acid reagent, forming an alkyloxonium ion. This ion acts as a very good leaving group which leaves to form a carbocation. The deprotonated acid then attacks an α-hydrogen adjacent to the carbocation and a double bond is formed. Charcoal boiling chips instead of regular boiling chips were used because phosphoric acid would dissolve the regular boiling chips. Therefore the synthesizing of cyclohexene from cyclohexanol goes through a major E1 reaction and no Sn1 reaction. An E1 reaction is favored in this case because the reaction takes place at high temperatures which strongly reduces the possibility of a substitution reaction happening. Since the OH- group is attached to a secondary carbon, the reaction could either go through an E1 or an E2 reaction. Looking at the reagent useds, phosphoric acid is a strong acid and its conjugate base (H2PO4−) is weak which acts as the non-nucleophile acid favoring E1 reactions with secondary and tertiary substrates and so the E1 mechanism triumphs over E2. A
Sn2 reaction does not occur because a relatively good nucleophile is needed for Sn2 and H2PO4− is a very poor nucleophile. A simple distillation apparatus was used to collect cyclohexene from cyclohexanol by monitoring the vapor temperature, making sure that it does not exceed 1000C to guarantee that the distillate collected was strictly cyclohexene. This is because cyclohexene has a lower boiling point (82.980C)2 than cyclohexanol (160.840C)2, so heating the mixture in the hood under low pressure over 1000C might cause cyclohexanol to begin to distill if it reaches its boiling point thus making the separation useless. The cyclohexene distillate was collected at a stable temperature of 75.50C. Conversely, if the reaction is not heated enough, the cyclohexanol molecules do not dehydrate to form alkenes but react with one another to form Sn1 products. Temperature helps to lower the activation energy and to make the ΔG for the E1 process very negative thereby favoring the reaction. The cyclohexene distillate was collected in a flask immersed in an iced bath, to cool the solution, to make it remain a liquid, thereby containing the liquid in the flask by preventing it from evaporating Aqueous sodium carbonate was added to the cyclohexene distillate to neutralize any phosphoric acid that distilled over into the receiving flask. Saturated sodium chloride was added to the cyclohexene mixture in the separatory funnel in two increments to separate the organic layer containing cyclohexene and the aqueous layer containing the neutralized acid and water produced from the dehydration. The saturated sodium chloride also minimizes the solubility of water in the organic layer making the organic and aqueous layer in the funnel even more immiscible making the separation possible. Anhydrous sodium sulfate was added to the cyclohexene fraction to dry any last traces of water left in the cyclohexene product. This step is very important because if the water is not removed from the product, when the IR for the product
is done, a false hydroxide peak will be seen which is indicative of an alcohol which in this case would be water because it will be present in the product. The bromine test done proved that the product was indeed an alkene as the deep brown color was seen to disappear completely to form a clear solution of the cyclohexene which indicates that a halogenation did take place at the alkene double bond. The mass of cyclohexene collected after the drying of the product with anhydrous sodium sulfate was 4. 73g and the percent yield of cyclohexene from cyclohexanol was 58.51%. The percent yield is low because not all of the cyclohexene was distilled from the simple distillation flask. Also, in decanting the cyclohexene product from the anhydrous sodium sulfate, some of the cyclohexene was left behind to prevent the inclusion of sodium sulfate in the dried cyclohexene product. Looking at the IR spectrum of cyclohexene, the peaks observed at 3061.69cm-1 and 3022.12cm-1 (letter A and B on IR spectrum) suggests the presence of a sp2 carbon bonded to a hydrogen atom, which is expected since the carbon atoms that bear the double bond in the cyclohexene molecule are indeed sp2 hybridized and are each bonded to one hydrogen atom. Other peaks observed at 2983.45cm-1, 2928.23cm-1 and 2837.41cm-1( letter C, D and E on IR spectrum) suggest the presence of Csp3-H bonds which equally makes sense since all the other carbons in the cyclic molecule are indeed sp3 hybridized, with each bonded to two hydrogen atoms. The peak at 1652.26cm-1 (letter G on IR spectrum) suggests the presence of an alkenyl group. This peak was also expected since cyclohexene is an alkenyl group. The peaks observed from 1437.61cm-1- 639.95cm-1 (letter H-Q on IR spectrum) is considered the fingerprint region of cyclohexene which is exclusive to the product because every molecule has a fingerprint region unique to them. No unexpected peaks showed up in the IR spectrum, indicating that the product is indeed cyclohexene and it is not contaminated with any impurities.
Works cited 1. Padias, A. B. Making the connections a how-to guide for organic chemistry lab techniques. Second. Plymouth: Hayden-McNeil Publishing, 2011. Print 2. "Compound Properties – PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information, n.d. web. 11 November 2012.
Abstract The goal of this experiment was to synthesize cyclohexene from cyclohexanol by unimolecular elimination (E1) through the dehydration of cyclohexanol (scheme1) Phosphoric acid was used to catalyze the reaction by providing the proton necessary for the conversion of the bad OH- group to water which is a good leaving group. The unimolecular elimination was favored over unimolecular substitution by heating the reaction at a high temperature and also by the use of the non-nucleophilic phosphoric acid. A bromine test and Infrared spectroscopy was done to test the presence of an alkene in the product formed by dehydration which indeed confirmed the presence of an alkene. The mass of cyclohexene collected from the dehydration was 4.73g and the percent yield of cyclohexene was 58.51%.
Introduction Alkenes are hydrocarbons which have carbon–carbon double bonds (σ bond and π bond) and are one of the many functional groups in organic molecules. Alkenes are sp2 hybridized and are unsaturated because they are missing two hydrogens from the saturated alkane formula (CnH2n+2). Typically alkenes are synthesized by elimination reactions. Apart from elimination reactions, organic molecules can also undergo substitution reactions amongst many other reactions like oxidation and reduction based on the structure of the organic molecule and the conditions under which the reaction is performed. All of these reactions are possible because the carbon atom in organic molecules is electron deficient due to induced dipoles created by the presence of functional groups or more electronegative atoms/groups. The organic molecule as a
whole is referred to as the substrate. The carbon atom in the organic molecule which is of interest to be attacked is referred to as the electrophile and it is usually bonded to a more electronegative atom/group such as a halide, oxygen or psuedohalide, called the leaving group. In a reaction, a leaving group leaves the substrate by heterolysis (a reaction in which the breaking of bonds leads to the formation of ion pairs) and it is replaced by a nucleophile (usually a base) which is attracted to the partial positive carbon atom because the nucleophile has excess electron pairs or a negative charge. The relative strength of a nucleophile determines its nucleophilicity. Nucleophilicity depends on many factors, including charge, basicity, solvent, polarizability, and the nature of the substituents present on the organic molecule. In general, a nucleophile containing a negatively charged reactive atom is better than a nucleophile containing a reactive atom that is neutral and basicity parallels nucleophilicity meaning a strong base is a good nucleophile and a weak base is a weak nucleophile. Also, there are several factors that contribute to the ability of a group/atom to function as a good leaving group which includes the property of the carbon-leaving group bond (polarizability and strength), and the stability of the leaving group. Weak bases are good leaving groups and strong bases are bad leaving groups. The substrate organic molecule containing the electrophile is characterized in terms of steric effect which entails the bulkiness of the substrate and its shape/size in 3D and whether the electrophilic carbon is a primary, secondary or tertiary carbon. The nature of the substrate, nucleophile, electrophile and solvent (polar protic or polar aprotic) used will determine whether an organic molecule will undergo a substitution or elimination reaction. An elimination reaction is one in which an α- hydrogen and a leaving group are removed from an organic molecule in either a one step or a two-step mechanism to form an alkene. The
one and two-step mechanisms are bimolecular (E2) and unimolecular (E1) elimination reactions respectively, defined based on the rates of the reactions. The rates of the reactions are based on the kinematics of each reaction and not on the number of steps in the reaction. As implied by the names, E2 reactions have a rate factor of two (second-order) because the rate of the reaction is based on the concentration of the substrate and the nucleophile while E1 reactions have a rate factor of one (first-order) because the rate of the reaction is based solely on the concentration of the substrate. E2 is a one-step concerted process with a single transition state typically undergone by primary and secondary substrates (scheme2) while E1 is a two-step process of elimination (carbocation formation and deprotonation) undergone by secondary and tertiary substrates only (scheme3). E2 requires a reasonably good nucleophile (strong base) and a polar protic solvent. E1 is favored by tertiary and secondary substrates structures with a bad nucleophile in the presence of a polar protic solvent in which the tertiary substrate is more reactive than the secondary due to greater stabilization of the carbocation and lower activation energy. The polar protic solvent also helps stabilize the carbocation. In an E1 reaction, the most substituted alkene (most stable) is preferred over the least substituted one according to Saytaev’s rule (scheme4). This is due to the stabilization of the carbocation intermediate formed by hyperconjugation which leads to rearrangements involving hydride shifts or alkyl shifts in which a hydrogen or an alkyl group with its pair of electrons relocate to the carbocation to form a more stable carbocation.
A substitution reaction is one in which a leaving group on a substrate is replaced by a nucleophile in either a one step or two step processes, which is a bimolecular substitution (Sn2) and unimolecular substitution (Sn1) respectively. Sn2 is undergone by primary and secondary substrates only preferable in a polar aprotic solvent with a relatively good nucleophile while Sn1 is undergone by secondary and tertiary substrates only preferably in a polar protic solvent with a poor nucleophile. Tertiary substrates are better than secondary substrates for Sn1 also because of the stabilization of the carbocation with the polar protic solvent increasing the stability of the carbocation. There is always competition between substitution and elimination reactions depending of the conditions of the reaction. Under normal conditions (room temperature, relative nucleophile, polar protic solvent etc.), Sn1 is always accompanied by E1 as the minor product but E1 products can be favored by changing the conditions of the reaction such as higher temperatures which favor E1 products. A common way to synthesize alkenes is by the dehydration of alcohols (scheme5). The reaction is acid catalyzed to transform the hydroxide (OH-) bad leaving group into a good leaving group (H2O) and so a basic nucleophile cannot be used because it cannot provide the proton needed to transform OH- to H2O. The acid used has to be non-nucleophilic such as phosphoric acid or sulfuric acid in a high concentration for an E1 product to be favored over a Sn1 product. High temperatures will also favor E1 over Sn1 due to the energetics of the reaction
as Gibbs free energy (ΔG) becomes significantly negative. The entropy of a Sn1 reaction is zero while the entropy of an E1 reaction is greater than zero, so increasing the temperature make ΔG even more negative increasing the spontaneity of E1 (scheme6).
To test the effectiveness of a reaction producing an alkene, the product of the reaction can be tested for the presence of an alkene using the bromine test or infrared spectroscopy. The bromine test occurs by halogenation at the double bond in which two bromine radicals break the double bond by binding to the two carbons in the double bond. The reaction occurs as a bromine radical attacks one of the sides of the p-orbital in a pi-bond forcing the bond to cleave homolytically and a bromine-carbon bond is formed at the point where the p-orbital of the carbon was attacked. The presence of an alkene is indicated by the disappearance of the deep brown coloration of bromine, which happens because the bromine has been consumed by the reaction of the unknown sample. Infrared spectroscopy (IR) is a technique by which a molecule is analyzed based on its absorbance of infrared light due to the functional groups present within the molecule in a single non-destructive test. IR is possible because every bond has a natural frequency and the amount of energy applied to a bond affects the amplitude of the vibration of a bond. Therefore it is possible to detect functional groups on an IR spectrum, as each functional group will have a specific natural frequency that is characteristic of that group, which corresponds to a specific absorption region of the spectrum. The natural vibration frequency of
any bond relies on its bond order (single, double, triple) and the bonded atoms such as hydrogen, oxygen or nitrogen as determined by Hooke’s law (scheme7). Hooke’s law gives a basic idea of how the bond between atoms will act, in comparison to a spring, with the two atoms acting like weights attached to the spring. By performing an IR spectroscopy on the product of a reaction, it is possible to discern whether a reaction took place, or if the right reaction took place, by analyzing the IR spectrum for that product, comparing the peaks to standard peaks. Alkenyl alkenes have a standard absorption frequency of 1620cm-1-1680cm-1 and aromatic alkenes have a standard absorption frequency of 1500cm-1&1600cm-1 and so these peaks (aromatic or alkenyl)1 have to be present in the IR spectrum of the product to indicate the presence of an alkene.
Reagent table 1&2 Name,
Molecular
Structure
weight(g/mol) used
Cyclohexanol 100.16
Cyclohexene
84.16
Amount Concentration Density(g/ml) Melting Boiling point
point
(0C)
(0C)
10mL
0.962
25.93
160.84
4.73g
0.811
-103.5
82.98
98.00
12mL
Bromine
79.90
Anhydrous
Phosphoric
85%
1.88
42.35
158
≈3drops
3.103
-7.2
58.8
142.04
≈2g
2.68
884
1429
58.44
20mL
2.16
801
1413
acid
NaSO4
NaCl
Experimental An amount of 10mL cyclohexanol, 12mL of 85% phosphoric acid and 3 charcoal boiling chips were added to a 100mL round bottom flask. A simple distillation apparatus was assembled with a 50mL round bottom flask immersed in an ice-water bath used as the receiving flask and the heating mantle was set to 55%. The temperature at which the distillate collected was recorded to be 75.50C . The distillate (cyclohexene) was transferred to a separatory funnel and 5mL of 10% Na2CO3 was added to it. The mixture was washed twice with 2 portions of 10mL saturated NaCl with the cyclohexene layer collected. The cyclohexene was placed in a 25ml Erlenmeyer flask and dry anhydrous NaSO4 was added to it to dry the product. The dried cyclohexene product was placed in a pre-weighed round bottom flask & beaker (167.82g) and weighed to be 172.55g. The mass of the cyclohexene produced was determined to be 4.73g and the percent
yield of cyclohexene was calculated to be 58.51%. A bromine test and IR spectroscopy was done for the product to test the presence of an alkene. Results Temperature at which stable distillate was collected=75.50C Mass of cyclohexene collected= Mass of beaker, round flask and product - mass of beaker and empty round bottom flask =172.55g-167.83g= 4.73g % Yield cyclohexene= Moles of cyclohexene X 100 Moles of cyclohexanol Moles of cyclohexanol=
Moles of cyclohexene=
% Yield of cyclohexene=
IR results Wavenumber (1/cm)
Bond
1437.65-639.95
fingerprint region
1652.26
Akenyl(C=C)
2837.41
C (sp3) –H
2928.23
C(sp3)-H
2983.45
C(sp3)-H
3022.12
C (sp2) –H
3061.69
C (sp2) –H
Cyclohexene is an oily clear liquid and when bromine was added to it, the color changed to deep brown. As the reaction proceeded, the deep brown color disappeared until the mixture became clear again. Discussion An alkene (cyclohexene) is synthesized in this experiment by the dehydration of an alcohol (cyclohexanol) through the loss of water to form a double bond (Scheme1). The dehydration proceeded by the heating of cyclohexanol in the presence of phosphoric acid at a high temperature. Phosphoric acid was used because it is a strong acid and so it can dissociate completely to form ions thereby providing the proton (H+) needed for the conversion of the bad OH- leaving group to water which is a good leaving group. The OH- group in the alcohol donates two electrons to H+ from the acid reagent, forming an alkyloxonium ion. This ion acts as a very good leaving group which leaves to form a carbocation. The deprotonated acid then attacks an α-hydrogen adjacent to the carbocation and a double bond is formed. Charcoal boiling chips instead of regular boiling chips were used because phosphoric acid would dissolve the regular boiling chips. Therefore the synthesizing of cyclohexene from cyclohexanol goes through a major E1 reaction and no Sn1 reaction. An E1 reaction is favored in this case because the reaction takes place at high temperatures which strongly reduces the possibility of a substitution reaction happening. Since the OH- group is attached to a secondary carbon, the reaction could either go through an E1 or an E2 reaction. Looking at the reagent useds, phosphoric acid is a strong acid and its conjugate base (H2PO4−) is weak which acts as the non-nucleophile acid favoring E1 reactions with secondary and tertiary substrates and so the E1 mechanism triumphs over E2. A
Sn2 reaction does not occur because a relatively good nucleophile is needed for Sn2 and H2PO4− is a very poor nucleophile. A simple distillation apparatus was used to collect cyclohexene from cyclohexanol by monitoring the vapor temperature, making sure that it does not exceed 1000C to guarantee that the distillate collected was strictly cyclohexene. This is because cyclohexene has a lower boiling point (82.980C)2 than cyclohexanol (160.840C)2, so heating the mixture in the hood under low pressure over 1000C might cause cyclohexanol to begin to distill if it reaches its boiling point thus making the separation useless. The cyclohexene distillate was collected at a stable temperature of 75.50C. Conversely, if the reaction is not heated enough, the cyclohexanol molecules do not dehydrate to form alkenes but react with one another to form Sn1 products. Temperature helps to lower the activation energy and to make the ΔG for the E1 process very negative thereby favoring the reaction. The cyclohexene distillate was collected in a flask immersed in an iced bath, to cool the solution, to make it remain a liquid, thereby containing the liquid in the flask by preventing it from evaporating Aqueous sodium carbonate was added to the cyclohexene distillate to neutralize any phosphoric acid that distilled over into the receiving flask. Saturated sodium chloride was added to the cyclohexene mixture in the separatory funnel in two increments to separate the organic layer containing cyclohexene and the aqueous layer containing the neutralized acid and water produced from the dehydration. The saturated sodium chloride also minimizes the solubility of water in the organic layer making the organic and aqueous layer in the funnel even more immiscible making the separation possible. Anhydrous sodium sulfate was added to the cyclohexene fraction to dry any last traces of water left in the cyclohexene product. This step is very important because if the water is not removed from the product, when the IR for the product
is done, a false hydroxide peak will be seen which is indicative of an alcohol which in this case would be water because it will be present in the product. The bromine test done proved that the product was indeed an alkene as the deep brown color was seen to disappear completely to form a clear solution of the cyclohexene which indicates that a halogenation did take place at the alkene double bond. The mass of cyclohexene collected after the drying of the product with anhydrous sodium sulfate was 4. 73g and the percent yield of cyclohexene from cyclohexanol was 58.51%. The percent yield is low because not all of the cyclohexene was distilled from the simple distillation flask. Also, in decanting the cyclohexene product from the anhydrous sodium sulfate, some of the cyclohexene was left behind to prevent the inclusion of sodium sulfate in the dried cyclohexene product. Looking at the IR spectrum of cyclohexene, the peaks observed at 3061.69cm-1 and 3022.12cm-1 (letter A and B on IR spectrum) suggests the presence of a sp2 carbon bonded to a hydrogen atom, which is expected since the carbon atoms that bear the double bond in the cyclohexene molecule are indeed sp2 hybridized and are each bonded to one hydrogen atom. Other peaks observed at 2983.45cm-1, 2928.23cm-1 and 2837.41cm-1( letter C, D and E on IR spectrum) suggest the presence of Csp3-H bonds which equally makes sense since all the other carbons in the cyclic molecule are indeed sp3 hybridized, with each bonded to two hydrogen atoms. The peak at 1652.26cm-1 (letter G on IR spectrum) suggests the presence of an alkenyl group. This peak was also expected since cyclohexene is an alkenyl group. The peaks observed from 1437.61cm-1- 639.95cm-1 (letter H-Q on IR spectrum) is considered the fingerprint region of cyclohexene which is exclusive to the product because every molecule has a fingerprint region unique to them. No unexpected peaks showed up in the IR spectrum, indicating that the product is indeed cyclohexene and it is not contaminated with any impurities.
Works cited 1. Padias, A. B. Making the connections a how-to guide for organic chemistry lab techniques. Second. Plymouth: Hayden-McNeil Publishing, 2011. Print 2. "Compound Properties – PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information, n.d. web. 11 November 2012.
