CHAPTER 3 - ORGANIC COMPOUNDS ALKANES AND ...
CHAPTER 3 - ORGANIC COMPOUNDS ALKANES AND CYCLOALKANES There are over 15 million organic compounds, each with its own physical and chemical properties. Can be classifies into families based on structure and then can roughly predict (within certain limits) the chemistry of the compounds. These structural features are called FUNCTIONAL GROUPS. See Table 3.1, pages 70-71 in text. MEMORIZE
THIS TABLE
HYDROCARBONS - Contain only carbon and hydrogen 1. Alkanes - only carbon and hydrogen and all single bonds. 2. Functional groups with C-C multiple bonds. a. Alkenes
C C
b. Alkynes
C C
c. Arenes
C C C
C C C
Carbon Singly Bonded to Electronegative Atoms 1. Alcohols R-OH 2. Ethers
R-O-R'
3. Amines
R-NH2, R2NH, R3N
4. Sulfides R-S-R' 5. Thiols
R-SH
6. Other - sulfones, sulfoxides Carbon Doubly Bonded to Oxygen
1
H 1. Aldehydes
R
C O
The C=O is then attached to only H or hydrocarbon function.
R 2. Ketones
R'
C O O
3. Caboxylic acids
C
O C
4. Ester
OH The C=O is attache to one H OR one hydrocarbon function and to another electronegative element.
OR O C
5. Amide
NH2 O
6. Acid halide
C
X
Other Functional Groups include (but not limited to) C with multiple bonds to other electronegative elements such as nitrogen and sulfur. Examples:
Imines C NH Nitriles R
C N
Give examples of all functional groups.
ALKANES - "Saturated Hydrocarbons" 2
"Aliphatic Hydrocarbons"
"Normal Alkanes" General Formula CnH2n+2 All compounds of this class have only single bonds. The hybridization on carbon is sp3. Carbon-carbon bonds are sp3-sp3 overlap to form sigma (s) bonds; carbonhydrogen bonds are sp3-s overlap to also form sigma bonds. "Straight"-chain alkanes:
CH4
C2 H6
C3 H8
H H C H H H C H
methane
H
+ H C H H
CH2
ethane +
H H H H C C C H H H H
HOMOLOGOUS CH2
SERIES
propane +
C4 H10
H H H H H C C C C H H H H
H
butane
C5 H12
H H H H H C C C C H H H H
H C H H
pentane
C6 H14
H H H H H C C C C H H H H
H H C C H H H
hexane
CH2
heptane
C7 H16
octane, nonane, decane, undecane, dodecane, tridecane, icosane, etc. Emphasize methods of shorthand writing 3
H H H H C C C H H H H
H H H H H H C C C C C H H H H H H
CH3CH 2CH3
CH3CH 2CH2CH 2CH3
Show not a straight chain. Example: BUTANE H3C
CH2 CH2
CH3
CH2 CH2
CH3
CH3 CH3
CH2 CH2 CH3
The suffix - ANE indicates that the compounds is an ALKANE. All alkanes will have this same suffix. Another preliminary note about naming (nomenclature)....... If a hydrogen is removed from an alkane, what remains is an ALKYL group. CH4 -------> CH3- and the name of the group is a METHYL group. CH3CH2- is an ethyl group....etc..... Name CH3-
Group Methyl
Abbreviation Me-
Example Methyl alcohol
CH3CH2-
Ethyl
Et-
Ethyl chloride
CH3CH2CH2-
Propyl
Pr-
Propyl amine (n-C3H7-)
Bu-
Butyl bromide (n-C4H9-)
CH3CH2CH2CH2- Butyl
Branched-chain Alkanes - ISOMERS 4
With propane and higher hydrocarbons, there are different KINDS of hydrogens in the molecules. CH3-CH2-CH3 a b a
CH3-CH2-CH2-CH2-CH3 a b c b a
Can generate different molecules by substituting groups (ex - a methyl group) for the different kinds of hydrogen.
H H H H C C C H H H
H H H H C C C H H H H
CH3CHCH 3 CH3
H C H H
ISOMERS
H H H H H C C C CH H H H H
CH3CH2CH2 CH3
NOMENCLATURE Methane CH4 Pentane CH3(CH2)3CH3
Ethane CH3CH3
Propane CH3CH2CH3
Isopentane (CH3)2CHCH2CH3
Butane (Isobutane) CH3CH2CH2CH3 CH3CHCH3 | CH3 Neopentane (CH3)4C
There are five (5) hexanes, nine(9) heptanes, 18 octanes. Obviously we must devise a systematic method of naming compounds so that everyone can understand exactly what you are talking about.
General Nomenclature: 5
All names consist of the following components: PREFIX -----------------PARENT ----------------SUFFIX Substituent and minor functional groups with positions indicated
Number of carbons in main chain
Indicates major functional group
Straight chain alkanes (no branches) named according to the number of carbons in the chain---no prefix (no substituents) and suffix is ANE as noted previously. Know C1 through C12 and C20 A few words about alkyl substituents...... We have seen methyl, ethyl, n-propyl, n-butyl... etc. Some complications arise with propyl and butyl groups...... CH3CH2CH3
Propane
CH3 CH CH3
CH3CH2CH2
Propyl
Isopropyl
CH3CH2CH2 CH3
CH3 CH2CH2CH2-
Butane
Butyl
CH3 CH3
CH CH3
Isobutane
CH3
CH3 CH2 CHCH3
2˚-Butyl (sec-Butyl) CH3 CH CH2
Isobutyl
CH3 H3C
C CH3
3˚-Butyl (tert-Butyl)
KNOW THESE!!!! Prefixes primary, secondary, tertiary and quaternary refer to the alkyl substitution on the carbon in question. H H R R R C H R C H R C H R C R H R 1˚ 2˚ 3˚ R 4˚ R
6
H3C
CH
CH3
CH3CH 2CH2CHCH2CH 2CH3 4-(1-Methylethyl)heptane OR 4-Isopropylheptane Rules of Nomenclature of Alkanes 1. a. Find the parent hydrocarbon Longest continuous chain of carbons (Remember, not necessarily drawn in a straight line). b. If two different chains of equal length, select one with the greatest number of substituents. CH3 CH3 CH CHCH2CH2CH 3 CH2CH 3
Two possible chains. Pick the one with 2 substituents rather than the one with one substituent.
2. a. Number the atoms in the main chain. Begin at the end nearest first branch so that the substituents have the lowest position numbers. CH3 CH2 CH3 CH CHCH2CH3 CH2CH 2CH3 b. If first branch is equidistant from both sides, begin nearest second branch. CH3CH 2 CH3
CH3 CH CH2CH2 CH CH CH2CH 3 CH2CH 3
3. a. Identify and number the substituents.
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3-methyl or 2-ethyl .....etc. b. Number must be repeated if two substituents are on the same carbon whether the substituents are the same or different. c. Cite in alphabetical order. d. If two or more identical substituents, use di-, tri-, tetra-, prefixes. Do NOT use these prefixes for alphabetizing. 4. Write out name as single word, using hyphen between words and numbers and commas between numbers. CH3CH 2 CH3
CH3 CH2CH 3
C CH2 CH2 CH CHCH2CH3 H
CH3 CH3CH CHCH2CH2CH 3
3-Ethyl-4,7-dimethylnonane
3-Ethyl-2-methylhexane
CH2CH 3 CH3CH 2
CH3 CHCH2CH2CCH3 CH2CH 2CH3
4,4-Dimethylnonane
5. (If Necessary) Complex Substituents Complex substituent is named by using the first 4 steps as if it were the compound itself. CH3 Molecule
CH2 CHCH3
Start at the point of attachment. Longest chain - 3 carbons, number 1,2,3 from point of attachment, with a methyl group on second carbon. Therefore 8
group is 2-methylpropyl substituent. This goes in parantheses with the number outside the parens indicating where this group is found on parent hydrocarbon. CH3 CH3CH CHCH2CH2 CHCH2CHCH3 CH3 CH3 CH2CH 2CH2CH 3 2,3-Dimethyl-6-(2-methylpropyl)decane Additional Notes: 1˚, 2˚, 3˚, 4˚ CARBONS and HYDROGENS. Called primary, secondary, tertiary and quarternary.
R
H C H 1˚
R
R C R 3˚
Also use symbol R- to represent generalized organic portion of the molecule. R-OH is an alcohol, where R = CH3- , or C2H5- or something larger and more complicated. Occurrence of Alkanes - Petroleum Many alkanes are naturally occurring. Major source is natural gas and petroleum products. Natural gas is methane, ethane, propane, butane and isobutane in respectively decreasing amounts. (Note that these are all colorless and ODORLESS and when you "smell" natural gas you are actually smelling an additive which contains the element sulfur). Petroleum is a complex mixture which can be refined by fractional distillation. Initial refining begins with three cuts - gasoline (30-200˚C), kerosene (175-300˚C)
9
and gas oil (275-400˚C). Under reduced pressure get lubricating oils, waxes, and tarry residue which is asphalt. Then these fractions are further refined. Antiknock gasoline - fuels that burn well. Octane number is measure of antiknock properties based on heptane = 0 and 2,2,4-trimethylpentane = 100. Straight-chain alkanes converted into aromatics by catalytic reforming. Kerosene is broken down by catalytic cracking. PROPERTIES OF ALKANES Called PARAFFINS from Latin meaning "slight affinity". These are relatively chemically inert. Show little chemical affinity for other molecules. Inert with most reagents used in organic chemistry. They do react with O2 under appropriate conditions. After all we use then as fuel!!! When ignited in the presence of oxygen will burn to form carbon dioxide and water, reactions which give off large amounts of heat. CH4 + 2O2 ----------> CO2 + 2H2O + 213kcal/mol Physical Properties: Regular increases in boiling point and melting with as molecular weight increases when comparing the straight-chain homologous series.. See Figure 3.4, p 87 in text. Branching lowers boiling point when compare compounds of same molecular weight (isomers). Pentane - 36.1˚C, isopentane - 27.8˚C, neopentane - 9.5˚C. Alkanes are non-polar compounds with only weak van der Waals attractions holding the molecules together. These forces only operate over small distances and are not uniform throughout the system. Temporary dipoles have fleeting existence. CYCLOALKANES 10
Carbon can bond to itself to form rings.. Called cycloalkanes or alicyclic compounds. Many are naturally occurring. Prostoglandins and steroids have alkane rings in their structures. Five- and six-memberd rings are most abundant in nature. Nomenclature of Cycloalkanes
(Systematic)
1. Use cycloalkane as the base (Parent) name, and place the prefix "CYCLO" in front of the alkane name. The only exceptions occur when the alkyl side chain contains a greater number of carbons than the ring OR the side chain would have a complicated name as a substituent. In these cases the ring is named as a substituent with the "yl" ending. CH3 CH2CH 2CH2CH 2CH3 1-Cyclopropylpentane
Methylcyclopentane
2. Number the positions on the ring so that the substituents have the lowest series of numbers possible.
1 6 5
CH3 1
2 3 CH3
4 1,3-Dimethylcyclohexane
2
CH3 6
3
5 CH3 4 NOT 1,5-Dimethylcyclohexane
a. When two or more alkyl groups are present, they are numbered alphabetically.
11
CH3 1-Ethyl-3-methylcyclopentane CH2CH 3 b. Halogen substituents are treated as are alkyl groups. Br 1-Bromo-3-ethylcyclopentane CH2CH 3 Cis-Trans Isomerism in Cycloalkanes Chemistry of cycloalkanes is similar to that of alkanes. Both are non-polar and relatively inert. Cycloalkanes have less flexibility than open-chain compounds. There is free rotation around carbon-carbon sigma bonds. (LOOK AT MODELS). Leads to many geometric arrangements of the hydrogens in the molecules. [We will study this further in the next chapter]. Much less freedom of rotation in rings. The sigma bonds in the ring cannot be fully rotated without breaking bonds. cis-1,2-Dibromocyclopentane is different from trans-1,2-dibromocyclopentane and they do not interconvert. Constitutional isomers - Butane and isobutane are constitutional isomers; i.e. the atoms are arranged in a different order. CH3 CH3CH 2CH2CH 3
CH3 CHCH3
12
In the two compounds mentioned above, the atoms are arranged in the same order of connectivity but have a different orientation in space. These compounds are called STEREOISOMERS. (A 3-D quality) Br
Br
Br
H
H
H
H
Br
13
THIS TABLE
HYDROCARBONS - Contain only carbon and hydrogen 1. Alkanes - only carbon and hydrogen and all single bonds. 2. Functional groups with C-C multiple bonds. a. Alkenes
C C
b. Alkynes
C C
c. Arenes
C C C
C C C
Carbon Singly Bonded to Electronegative Atoms 1. Alcohols R-OH 2. Ethers
R-O-R'
3. Amines
R-NH2, R2NH, R3N
4. Sulfides R-S-R' 5. Thiols
R-SH
6. Other - sulfones, sulfoxides Carbon Doubly Bonded to Oxygen
1
H 1. Aldehydes
R
C O
The C=O is then attached to only H or hydrocarbon function.
R 2. Ketones
R'
C O O
3. Caboxylic acids
C
O C
4. Ester
OH The C=O is attache to one H OR one hydrocarbon function and to another electronegative element.
OR O C
5. Amide
NH2 O
6. Acid halide
C
X
Other Functional Groups include (but not limited to) C with multiple bonds to other electronegative elements such as nitrogen and sulfur. Examples:
Imines C NH Nitriles R
C N
Give examples of all functional groups.
ALKANES - "Saturated Hydrocarbons" 2
"Aliphatic Hydrocarbons"
"Normal Alkanes" General Formula CnH2n+2 All compounds of this class have only single bonds. The hybridization on carbon is sp3. Carbon-carbon bonds are sp3-sp3 overlap to form sigma (s) bonds; carbonhydrogen bonds are sp3-s overlap to also form sigma bonds. "Straight"-chain alkanes:
CH4
C2 H6
C3 H8
H H C H H H C H
methane
H
+ H C H H
CH2
ethane +
H H H H C C C H H H H
HOMOLOGOUS CH2
SERIES
propane +
C4 H10
H H H H H C C C C H H H H
H
butane
C5 H12
H H H H H C C C C H H H H
H C H H
pentane
C6 H14
H H H H H C C C C H H H H
H H C C H H H
hexane
CH2
heptane
C7 H16
octane, nonane, decane, undecane, dodecane, tridecane, icosane, etc. Emphasize methods of shorthand writing 3
H H H H C C C H H H H
H H H H H H C C C C C H H H H H H
CH3CH 2CH3
CH3CH 2CH2CH 2CH3
Show not a straight chain. Example: BUTANE H3C
CH2 CH2
CH3
CH2 CH2
CH3
CH3 CH3
CH2 CH2 CH3
The suffix - ANE indicates that the compounds is an ALKANE. All alkanes will have this same suffix. Another preliminary note about naming (nomenclature)....... If a hydrogen is removed from an alkane, what remains is an ALKYL group. CH4 -------> CH3- and the name of the group is a METHYL group. CH3CH2- is an ethyl group....etc..... Name CH3-
Group Methyl
Abbreviation Me-
Example Methyl alcohol
CH3CH2-
Ethyl
Et-
Ethyl chloride
CH3CH2CH2-
Propyl
Pr-
Propyl amine (n-C3H7-)
Bu-
Butyl bromide (n-C4H9-)
CH3CH2CH2CH2- Butyl
Branched-chain Alkanes - ISOMERS 4
With propane and higher hydrocarbons, there are different KINDS of hydrogens in the molecules. CH3-CH2-CH3 a b a
CH3-CH2-CH2-CH2-CH3 a b c b a
Can generate different molecules by substituting groups (ex - a methyl group) for the different kinds of hydrogen.
H H H H C C C H H H
H H H H C C C H H H H
CH3CHCH 3 CH3
H C H H
ISOMERS
H H H H H C C C CH H H H H
CH3CH2CH2 CH3
NOMENCLATURE Methane CH4 Pentane CH3(CH2)3CH3
Ethane CH3CH3
Propane CH3CH2CH3
Isopentane (CH3)2CHCH2CH3
Butane (Isobutane) CH3CH2CH2CH3 CH3CHCH3 | CH3 Neopentane (CH3)4C
There are five (5) hexanes, nine(9) heptanes, 18 octanes. Obviously we must devise a systematic method of naming compounds so that everyone can understand exactly what you are talking about.
General Nomenclature: 5
All names consist of the following components: PREFIX -----------------PARENT ----------------SUFFIX Substituent and minor functional groups with positions indicated
Number of carbons in main chain
Indicates major functional group
Straight chain alkanes (no branches) named according to the number of carbons in the chain---no prefix (no substituents) and suffix is ANE as noted previously. Know C1 through C12 and C20 A few words about alkyl substituents...... We have seen methyl, ethyl, n-propyl, n-butyl... etc. Some complications arise with propyl and butyl groups...... CH3CH2CH3
Propane
CH3 CH CH3
CH3CH2CH2
Propyl
Isopropyl
CH3CH2CH2 CH3
CH3 CH2CH2CH2-
Butane
Butyl
CH3 CH3
CH CH3
Isobutane
CH3
CH3 CH2 CHCH3
2˚-Butyl (sec-Butyl) CH3 CH CH2
Isobutyl
CH3 H3C
C CH3
3˚-Butyl (tert-Butyl)
KNOW THESE!!!! Prefixes primary, secondary, tertiary and quaternary refer to the alkyl substitution on the carbon in question. H H R R R C H R C H R C H R C R H R 1˚ 2˚ 3˚ R 4˚ R
6
H3C
CH
CH3
CH3CH 2CH2CHCH2CH 2CH3 4-(1-Methylethyl)heptane OR 4-Isopropylheptane Rules of Nomenclature of Alkanes 1. a. Find the parent hydrocarbon Longest continuous chain of carbons (Remember, not necessarily drawn in a straight line). b. If two different chains of equal length, select one with the greatest number of substituents. CH3 CH3 CH CHCH2CH2CH 3 CH2CH 3
Two possible chains. Pick the one with 2 substituents rather than the one with one substituent.
2. a. Number the atoms in the main chain. Begin at the end nearest first branch so that the substituents have the lowest position numbers. CH3 CH2 CH3 CH CHCH2CH3 CH2CH 2CH3 b. If first branch is equidistant from both sides, begin nearest second branch. CH3CH 2 CH3
CH3 CH CH2CH2 CH CH CH2CH 3 CH2CH 3
3. a. Identify and number the substituents.
7
3-methyl or 2-ethyl .....etc. b. Number must be repeated if two substituents are on the same carbon whether the substituents are the same or different. c. Cite in alphabetical order. d. If two or more identical substituents, use di-, tri-, tetra-, prefixes. Do NOT use these prefixes for alphabetizing. 4. Write out name as single word, using hyphen between words and numbers and commas between numbers. CH3CH 2 CH3
CH3 CH2CH 3
C CH2 CH2 CH CHCH2CH3 H
CH3 CH3CH CHCH2CH2CH 3
3-Ethyl-4,7-dimethylnonane
3-Ethyl-2-methylhexane
CH2CH 3 CH3CH 2
CH3 CHCH2CH2CCH3 CH2CH 2CH3
4,4-Dimethylnonane
5. (If Necessary) Complex Substituents Complex substituent is named by using the first 4 steps as if it were the compound itself. CH3 Molecule
CH2 CHCH3
Start at the point of attachment. Longest chain - 3 carbons, number 1,2,3 from point of attachment, with a methyl group on second carbon. Therefore 8
group is 2-methylpropyl substituent. This goes in parantheses with the number outside the parens indicating where this group is found on parent hydrocarbon. CH3 CH3CH CHCH2CH2 CHCH2CHCH3 CH3 CH3 CH2CH 2CH2CH 3 2,3-Dimethyl-6-(2-methylpropyl)decane Additional Notes: 1˚, 2˚, 3˚, 4˚ CARBONS and HYDROGENS. Called primary, secondary, tertiary and quarternary.
R
H C H 1˚
R
R C R 3˚
Also use symbol R- to represent generalized organic portion of the molecule. R-OH is an alcohol, where R = CH3- , or C2H5- or something larger and more complicated. Occurrence of Alkanes - Petroleum Many alkanes are naturally occurring. Major source is natural gas and petroleum products. Natural gas is methane, ethane, propane, butane and isobutane in respectively decreasing amounts. (Note that these are all colorless and ODORLESS and when you "smell" natural gas you are actually smelling an additive which contains the element sulfur). Petroleum is a complex mixture which can be refined by fractional distillation. Initial refining begins with three cuts - gasoline (30-200˚C), kerosene (175-300˚C)
9
and gas oil (275-400˚C). Under reduced pressure get lubricating oils, waxes, and tarry residue which is asphalt. Then these fractions are further refined. Antiknock gasoline - fuels that burn well. Octane number is measure of antiknock properties based on heptane = 0 and 2,2,4-trimethylpentane = 100. Straight-chain alkanes converted into aromatics by catalytic reforming. Kerosene is broken down by catalytic cracking. PROPERTIES OF ALKANES Called PARAFFINS from Latin meaning "slight affinity". These are relatively chemically inert. Show little chemical affinity for other molecules. Inert with most reagents used in organic chemistry. They do react with O2 under appropriate conditions. After all we use then as fuel!!! When ignited in the presence of oxygen will burn to form carbon dioxide and water, reactions which give off large amounts of heat. CH4 + 2O2 ----------> CO2 + 2H2O + 213kcal/mol Physical Properties: Regular increases in boiling point and melting with as molecular weight increases when comparing the straight-chain homologous series.. See Figure 3.4, p 87 in text. Branching lowers boiling point when compare compounds of same molecular weight (isomers). Pentane - 36.1˚C, isopentane - 27.8˚C, neopentane - 9.5˚C. Alkanes are non-polar compounds with only weak van der Waals attractions holding the molecules together. These forces only operate over small distances and are not uniform throughout the system. Temporary dipoles have fleeting existence. CYCLOALKANES 10
Carbon can bond to itself to form rings.. Called cycloalkanes or alicyclic compounds. Many are naturally occurring. Prostoglandins and steroids have alkane rings in their structures. Five- and six-memberd rings are most abundant in nature. Nomenclature of Cycloalkanes
(Systematic)
1. Use cycloalkane as the base (Parent) name, and place the prefix "CYCLO" in front of the alkane name. The only exceptions occur when the alkyl side chain contains a greater number of carbons than the ring OR the side chain would have a complicated name as a substituent. In these cases the ring is named as a substituent with the "yl" ending. CH3 CH2CH 2CH2CH 2CH3 1-Cyclopropylpentane
Methylcyclopentane
2. Number the positions on the ring so that the substituents have the lowest series of numbers possible.
1 6 5
CH3 1
2 3 CH3
4 1,3-Dimethylcyclohexane
2
CH3 6
3
5 CH3 4 NOT 1,5-Dimethylcyclohexane
a. When two or more alkyl groups are present, they are numbered alphabetically.
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CH3 1-Ethyl-3-methylcyclopentane CH2CH 3 b. Halogen substituents are treated as are alkyl groups. Br 1-Bromo-3-ethylcyclopentane CH2CH 3 Cis-Trans Isomerism in Cycloalkanes Chemistry of cycloalkanes is similar to that of alkanes. Both are non-polar and relatively inert. Cycloalkanes have less flexibility than open-chain compounds. There is free rotation around carbon-carbon sigma bonds. (LOOK AT MODELS). Leads to many geometric arrangements of the hydrogens in the molecules. [We will study this further in the next chapter]. Much less freedom of rotation in rings. The sigma bonds in the ring cannot be fully rotated without breaking bonds. cis-1,2-Dibromocyclopentane is different from trans-1,2-dibromocyclopentane and they do not interconvert. Constitutional isomers - Butane and isobutane are constitutional isomers; i.e. the atoms are arranged in a different order. CH3 CH3CH 2CH2CH 3
CH3 CHCH3
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In the two compounds mentioned above, the atoms are arranged in the same order of connectivity but have a different orientation in space. These compounds are called STEREOISOMERS. (A 3-D quality) Br
Br
Br
H
H
H
H
Br
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