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Topic 5B
Bonding in carbon compounds
sp3 hybridization
• This is the reason why carbon is tetrahedral in many compounds
• By hybridization of its valence atomic orbitals, carbon can bond in a variety of ways
9
l=1
l=0
ml = -1 0 1
Valence shell 2s2 2p2
2s
2p
E px py pz
s
n=2
First look at the normal electronic configuration of carbon:
sp3 hybridization
• Promote one 2s electron into the vacant p-orbital.
• Combine (mix) all four orbitals to give four hybrid orbitals of equivalent energy:
9
2sp3
2s
2p
E
2p
2s
E
sp3 hybridization
• Each sp3 hybrid orbital has 25% “s” and 75% “p” character
• Each sp3 hybrid orbital looks like a distorted dumbell:
sp3hybrid2p2s
+
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sp3 Hybridization Animation
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The best arrangement of orbitals is a tetrahedral geometry making angles of 109°
Movie from Saunders General Chemistry CD-ROM
Tetrahedral bonding 10
C
H
H
H
H
methane
C
H
HH
H
H
C
HH
H
109.5°
• Each sp3 hybrid orbital has one electron and can form a strong covalent bond with another atom, eg methane formation with four hydrogens:
Sigma () bonds
• The H 1s and carbon sp3 hybrid orbitals are no longer separate entities and combine to form a sigma () bonding molecular orbital.
• These bonds are 109.5° apart.
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H
HH
H
C
109.5°
Sigma () bondformation
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Movie from Saunders General Chemistry CD-ROM
Other representations
Ball and stick Space Filling
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Other representations
Space Filling Potential Energy
Surface
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C–C bond formationin Ethane
Sigma () bonds can be formed between two carbons by overlapping two sp3 hybrid orbitals.
11
C
H
H
H
C
H
H
H
+ C
H
H
H
C
H
H
H
sp3 - sp3 bondbetween carbons
H
C
H
H C
H
H
H
Ethane
Ethane 11
Ball and stick Space filling
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Ethane 11Ethane can spin about the C—C bond
There is nearly free rotation:
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Propane 11Propane is formed by covalent bonding to two
other carbons and eight hydrogens.
Ball and stick Space filling
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Propane
Propane can rotate about both C—C bonds
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Butane 7Butane is formed by covalent bonding between
four carbons and ten hydrogens.
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Ball and stick Space filling
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Butane 7
Butane can rotate about all three C—C bonds
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Bonding to other atoms
• Alcohols are formed between sp3 hybridised carbon and oxygen:
C
H
H
H
O
H
O
H
C
H
H
H
+
sp3 - sp3 valence bondbetween carbon and oxygen giving an alcohol
12
Bonding to other atoms
• Amines are formed between sp3 hybridised carbon and nitrogen:
C
H
H
H
N
H
+ H
sp3 - sp3 valence bondbetween carbon and nitrogen giving an amine
N
H
C
H
H
H
H
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sp2 HybridizationDouble bond formation
• Carbon can form double bonds with itself and other heteroatoms.
• This requires sp2 hybridization of its valence atomic orbitals.
• Carbon is sp2 hybridized in:
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C C
H H
H HC O
H
H
Ethene (carbon sp2)
Formaldehyde(carbon, oxygen sp2)
sp2 Hybridization
• Promote one 2s electron into the vacant p-orbital.
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zyx
2p
2s
2p z
2sp22s
2p
E
combine
• Combine (mix) the 2s, 2px and 2py orbitals to give three hybrid orbitals of equivalent energy
• The 2pz orbital is unaltered.
E
sp2 Hybridization
• Only the 2px and 2py combine with the 2s orbital.
• The three hybrid orbitals make angles of 120° to minimise electron repulsion between them.
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2s 2py 2px
3 sp2 hybrid
orbitals
120° 120°
120°
Trigonal planar carbon
• There are four electrons — one in each orbital
• Note that the 2pz orbital is unchanged and perpendicular to the plane of the hybrid system.
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An sp2 hybridised carbonatom.
sp2 hybrid
sp2 hybrid
sp2 hybridC2pz
120°
120°
120°
Pi () bondingEthylene
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• Two sp2 carbons can form a covalentbond.
• Other hybrid orbitals covalently bond to four hydrogens.
C2pz C2pz
Pi () bondingEthene
14
• Less efficient sideways overlap of the pz orbitals gives a second C—C bond — a pi () bond.
• Both clouds (shown in green and blue) are part of the same -bonding orbital.
C2pz C2pz
H H
H HCH bonds CH bonds
CC bondH H
H H
CC bond
Pi () bondingEthene animation
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Movies from Saunders General Chemistry CD-ROM
Pi () bonding(theoretical approach) 14
• Overlap of two C2pz atomic orbitals forms two pi molecular orbitals, (lower in energy) and * (higher in energy).
• The electrons in C2pz orbitals are stabilised by occupying the lower energyorbital. p
One *-molecularorbital
E
*
C2pzC2pz
H
H H One -molecularorbital
H
H
H H
H
EthyleneBecause each carbon is trigonal planar, ethylene is a flat
molecule with thickness due to the pi-electrons.
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EthyleneThe pi-bond restricts rotation about the C=C bond.
A little twisting is possible but it is essentially rigid.
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EthyleneGeometry of ethylene:
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C C
H
H
H
H
121˚
118˚
134 pm
CH3 CH3154 pm
C NH
H H
C OH
H
Formaldehyde An imine
Other double bonded systems:
sp HybridizationAlkyne formation
• Carbon can form triple bonds with itself and with other heteroatoms (eg in H—C.
• This requires sp hybridization of its valence atomic orbitals.• Carbon is sp hybridized in ethyne, also called acetylene:
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Ethyne (carbon sp)
H C C H
sp Hybridization
• Promote one 2s electron into the vacant p-orbital.
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2s
2p
zyx
2p
2s
E
2sp
2py 2pz
combine
• Combine (mix) the 2s and 2px orbitals to give two hybrid orbitals of equivalent energy
• The 2py and 2pz orbital are unaltered.
E
sp Hybridization
• Only the 2px combines with the 2s orbital.
• The two hybrid orbitals make angles of 180° to minimise electron repulsion between them.
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2s 2px Two colinear sp hybridorbitals
180°
sp hybridised carbon
• The two hybrid orbitals are semi-occupied
• Note that the 2pz and 2py orbitals are unchanged and perpendicular to the plane of the hybrid system.
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An sp hybridised carbon atom
sp hybrid sp hybrid
C2py
C2pz
Triple bonding inEthyne
16
• Two sp hybridised carbons can form a covalentbond.
• Other hybrid orbitals covalently bond to two hydrogens.
C2pz C2pz
C2py C2py
Pi () bondingin Ethyne
16
• Less efficient sideways overlap of the pz and py orbitals
gives two C—C pi () bonds .
• These together with the bond form the triple bond.
C2pz C2pz
CH bond CH bond
CC bond
C2py C2py
• Two sets of clouds (shown in green and blue) form y and z
bonding orbital.
C C HH
y
z
Ethyne (acetylene)• Because each carbon is sp hybridised (hybrid
orbitals 180° apart) , ethyne is a linear molecule.
• Pi bonds form a barrel of electron density around the CC bond.
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Bond length—strengthCC bonds
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• Bond length decreases from single to double to triple bond.
• Bond strength increases from single to double to triple bond.
Summary:pm
Functional GroupsAlcohols
18
CH3OH
Methanol
O
H
C
H
H
H
H3C O
H
Functional GroupsAlcohols
18
CH3CH2OH
R OHR = alkyl group,OH = hydroxyl group
..
..
Ethanol
Functional GroupsAlcohols
18
CH2 OHR
CH OHR
R
C OHR
R
R
Primary (1°)
Secondary (2°)
Tertiary (3°)
One R group
Two R groups
Three R groups
Classification:
Functional GroupsAmines
19
• In methylamine, sp3 nitrogen is covalently bonded to methyl and two hydrogens
N
CH3H
HN
HC
HH H
H
Methylamine (Methanamine)
Functional GroupsAmines
19
• Classified on number of alkyl groups attached to nitrogen
R NH2 1 hydrogen replaced Primary (1°) amine
NR R'
H
Secondary (2°) amine2 hydrogens replaced
NR R'
Tertiary(3°) amine3 hydrogens replacedR''
Functional GroupsKetones and Aldehydes
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R CHOR = organic group,CHO = aldehyde group
C H
O
R
R = organic groups,CO = ketonic group
C R
O
R CO R
R
Functional GroupsKetones and Aldehydes
Formation of a bond using an sp2 hybrid orbital and a bond using the pz enables oxygen to form double bonds to carbon:
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C O
H
H
Polarised -molecular orbital
:
:
O2pzC2pz
CO -bond
H
H
120°
Functional GroupsKetones and Aldehydes 20
• Carbon is positively polarised and oxygen negatively polarised• Carbonyls are best seen as:
C Oδ+ δ–
Functional GroupsCarboxylic acids
21
R CO2HR = alkyl group,CO2H = carboxyl group
C OH
O
Functional GroupsCarboxylic acids
21
• Why acidic?• In water they ionize partially
R CO
O H
H
H
O R CO
O+ H3O
Carboxylate anion
(Hydronium ion)
Ka
Ka = [RCO2-][H3O+]
[RCO2H] pKa = -log Ka
Functional GroupsCarboxylic acids
21
• Resonance:• Negative charge is on both oxygens
R CO
OR C
O
O
Resonance hybrid
R C
O
O
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