What is Organic Chemistry?
What is Organic Chemistry? Study of compounds containing carbon Organic compounds also contain H, N, O, P, S, Cl…
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Organic Chemistry was first (and wrongly!) defined in 1770: • Compounds from living organisms • contain “vital force” that can’t be made in the lab Organic sugars plant oils starches waxes urea
Inorganic salts rocks gases
Vitalism disproved in 1828 Wöhler made urea (found in urine) from an inorganic salt:
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Why is Carbon special? Not very electronegative (shares well, makes strong covalent bonds) Can make long C-C chains or rings (biological molecules can have from 1 C to millions!) C can make up to four bonds.
http://www.columbia.edu/cu/opg/images/dna.jpg
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Examples of Organic Compounds
penic illin
Medicines Polymers Fuels Pesticides Dyes indigo
From Nature or Made in the Lab! Nylon 4
Drawing organic molecules
Dashed lines indicate a bond sticking back behind the plane.
Lines indicate a bond in the plane.
Wedges indicate a bond sticking out in front of the plane.
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Abbreviated Structures
pentane
Each vertex is a carbon. Each unspecified valence is a hydrogen. Each line segment represents a C at each end.
CH3CH3
H’s are omitted, but we know that C has 4 bonds so we can figure it out. We do not put a “C” where we know there is a carbon atom. 6
Zingerone
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Chapter 1: Structure and bonding
Nature of the atom: hydrogen and hydrogen-like orbitals
Bonding in molecules: covalent bonds, ionic bonds
Lewis Structures/VSEPR: bonding, shapes of molecules
Valence bond theory
Hybridization: sp3, sp2, sp
Read 1.11 – it will help you later, but we probably don’t have time
to discuss in class.
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Structure of Atom
Atom -neutral overall - positively charged central nucleus (containing positively charged protons and neutral neutrons) - nucleus surrounded by negatively charged electrons. Elements - atoms of each element have the same number of protons (and electrons) Element symbol
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Electronic Structure Quantum mechanics – uses mathematical equations to characterize the WAVE motion of an electron around the nucleus, called a wave equation, Ψ MORE IN CHM 139! The solutions to the wave equation (Schrödinger equation) gives orbitals – gives us energy and volume of space were electron is MOST LIKELY to be found. Orbitals of the same energy are called shells. 1st shell: s orbital 2nd shell: s orbital and 3 p orbitals 3rd shell: s orbital, 3 p orbitals and 5 d orbitals
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Energy of Orbitals For one-electrons, the energy is given by the shell only (principle quantum number, n). For polyelectronic atoms, the energy of the level depends on the level and type of orbital. (n and l)
Na
Rules for filling electrons: 1) Lowest energy orbitals fill first 2) 2 electrons/orbital with opposing spin 3) Electrons spaces themselves out before pairing (if two or more orbitals are available with equal energy, put one electron in each until all the orbitals are half full.) 11
The Periodic Table
Valence electrons – electrons in the outermost shell How many valence electrons for S?
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BONDING Bonds forms because the molecule formed has a lower energy than the atoms apart. Generally, electrons are shared or transferred so that each atoms has a filled valence shell.
Valence electrons are important for bonding. 13
Ionic Bonding:
Electrostatic forces between oppositely charged ions formed form
electron transfer.
unpaired electron, filled valence shell
unfilled valence shell
low high ionic bond ionization electron
energy affinity
electron transfer
Ionic compounds form because
opposites attract. Coulombic
attraction provide enough energy to
cause salt formation.
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Covalent Bonding: Electrons shared between atoms. unpaired electron, unfilled valence shell
electron sharing
filled valence shells
covalent bond
Why?
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Lewis Structures: Show valence electrons arranged among atoms. Model for bonding. Procedure: 1. Determine total number of valence electrons. 2. Draw covalent bond (2 electrons) between each atom and the central atom. 3. Give out the remaining electrons as lone-pairs so atoms have a full valence shell. 4. If atoms do not have full valence shell, use lone-pairs to form multiple bonds.
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Shape of Molecules Valence Shell Electron-Pair Repulsion Model Based on minimizing repulsions between electron groups on central atom Electron group = bond pair, multiple bond group, lone-pair of electrons
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MODELS OF BONDING Valence bond (VB or localized electron) theory Bonds form from overlap of atomic orbitals (each containing one electron.)
+ 1s
H 1s
H
sigma bond
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What about methane, CH4? C needs to form 4 bonds with H. Need 4 unpaired electrons (to pair with H 1s1 electrons)
C 2s __ 2p __ __ __
C 2s __ 2p __ __ __
• 4 bonds with 4 different orbitals • Would end up with different bond lengths and angles would be 90º
Methane – bonds are equal in length and 109.5º. 19
Take 4 orbitals of C, throw into blender to mix and get 4 new HYBRID orbitals.
C 2s __ 2p __ __ __
C sp3 __ __ __ __
Four sp3 hybrid orbitals (each orbital has the same characteristics) that form 4 sigma (single) bonds to 1s orbitals of the H atoms. This gives us 4 bonds that are equal in length with 109.5º bond angles!
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Hybridized orbitals in ethylene
C 2s __ 2p __ __ __
C 2s __ 2p __ __ __ C sp2 __ __ __ p __
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The two C of acetylene and the C of CO2 are sp hybridized.
linear
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Shortcut: number of electron groups = number of letters The N in ammonia is sp3 hybridized. tetrahedral
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ETHANE
ETHYLENE
ACETYLENE 180o
bond angle
109.5o
bond length
154 pm
133 pm
120 pm
bond energy
376 kJ/mol
611 kJ/mol
835 kJ/mol
120o
The double bond in ethylene is not quite twice as strong as the single bond. C-H bond energy
420 kJ/mol
444 kJ/mol
552 kJ/mol
The C-H bond increases in strength: sp3 < sp2 < sp.
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