Topic 5B Bonding in carbon compounds. sp 3 hybridization This is the reason why carbon is...

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

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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:

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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.

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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

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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

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• 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

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• Two sp hybridised carbons can form a covalentbond.

• Other hybrid orbitals covalently bond to two hydrogens.

C2pz C2pz

C2py C2py

Pi () bondingin Ethyne

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• 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

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CH3OH

Methanol

O

H

C

H

H

H

H3C O

H

Functional GroupsAlcohols

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CH3CH2OH

R OHR = alkyl group,OH = hydroxyl group

..

..

Ethanol

Functional GroupsAlcohols

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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

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• 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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