1. Topic 5B Bonding in carbon compounds

2. 9
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
First look at the normal electronic configuration of carbon:
l=1
l=0
ml =
Valence shell 2s2 2p2 2s 2p E p x y z s
n=2

3. 9
sp3 hybridization
Promote one 2s electron into the vacant p-orbital.
Combine (mix) all four orbitals to give four hybrid orbitals of equivalent energy: E

4. 9
sp3 hybridization
Each sp3 hybrid orbital has 25% “s” and 75% “p” character
Each sp3 hybrid orbital looks like a distorted dumbell:

5. sp3 Hybridization Animation9
sp3 Hybridization Animation
Movie from Saunders General Chemistry CD-ROM
The best arrangement of orbitals is a tetrahedral geometry making angles of 109°

6. 10
Tetrahedral bonding
Each sp3 hybrid orbital has one electron and can form a strong covalent bond with another atom, eg methane formation with four hydrogens:

7. 10
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.

8. Sigma () bond formation10
Sigma () bond formation
Movie from Saunders General Chemistry CD-ROM

9. Other representations10
Other representations
Ball and stick Space Filling

10. Other representations10
Other representations
Space Filling Potential Energy
Surface

11. C–C bond formation in Ethane11
C–C bond formation in Ethane
Sigma () bonds can be formed between two carbons by overlapping two sp3 hybrid orbitals. C H sp 3
- sp
bond
between carbons H H H C + C H H H

12. 11
Ethane
Ball and stick Space filling

13. Ethane 11 Ethane can spin about the C—C bond
There is nearly free rotation:

14. Propane 11 Propane is formed by covalent bonding to two
other carbons and eight hydrogens.
Ball and stick Space filling

15. 11
Propane
Propane can rotate about both C—C bonds

16. Butane 11 7 Butane is formed by covalent bonding between
four carbons and ten hydrogens.
Ball and stick Space filling

17. 11 7
Butane
Butane can rotate about all three C—C bonds

18. 12
Bonding to other atoms
Alcohols are formed between sp3 hybridised carbon and oxygen: H H H H H C + O H C O H H sp 3
- sp
valence bond
between carbon and
oxygen giving an alcohol

19. 12
Bonding to other atoms
Amines are formed between sp3 hybridised carbon and nitrogen: sp 3
- sp
valence bond
between carbon and
nitrogen giving an amine N H C H H H C + N H H

20. sp2 Hybridization Double bond formation13
sp2 Hybridization Double 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: H H H C C C O H H H
Ethene
(carbon sp2)
Formaldehyde
(carbon, oxygen sp2)

21. sp2 Hybridization 13 E combine
Promote one 2s electron into the vacant p-orbital.
Combine (mix) the 2s, 2px and 2py orbitals to give three hybrid orbitals of equivalent energy
The 2pz orbital is unaltered. E 2s 2p E z y x 2p 2s
combine 2p z
2sp 2

22. sp2 Hybridization 13 Only the 2px and 2py combine with the 2s orbital.
The three hybrid orbitals make angles of 120° to minimise electron repulsion between them.
3 sp 2
hybrid
orbitals
120° 2s 2p y 2p x

23. Trigonal planar carbon13
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.
C2p z sp 2
hybrid
120°
An sp 2
hybridised carbon
atom.
120° sp 2
hybrid
120° sp 2
hybrid

24. Pi () bonding Ethylene13
Pi () bonding Ethylene
Two sp2 carbons can form a covalentbond.
Other hybrid orbitals covalently bond to four hydrogens.
C2p z
C2p z

25. 14
Pi () bonding Ethene
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.
C2p z H CH s
bonds CC
bond H CC p
bond

26. Pi () bonding Ethene animation14
Pi () bonding Ethene animation
Movies from Saunders General Chemistry CD-ROM

27. 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 energyorbital. p
One *-molecular
orbital E  *
C2pz H
One  -molecular

28. Ethylene 14 Because each carbon is trigonal planar, ethylene is a flat
molecule with thickness due to the pi-electrons.

29. Ethylene 14 The pi-bond restricts rotation about the C=C bond.
A little twisting is possible but it is essentially rigid.

30. 15
Ethylene
Geometry of ethylene:
Other double bonded systems:

31. sp Hybridization Alkyne formation15
sp Hybridization Alkyne 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:
Ethyne
(carbon sp)

32. sp Hybridization 15 E combine
Promote one 2s electron into the vacant p-orbital.
Combine (mix) the 2s and 2px orbitals to give two hybrid orbitals of equivalent energy
The 2py and 2pz orbital are unaltered. E E z y x 2p 2s
combine 2p 2s

33. sp Hybridization 15 Only the 2px combines with the 2s orbital.
The two hybrid orbitals make angles of 180° to minimise electron repulsion between them.
180° 2s 2p x
Two colinear sp hybrid
orbitals

34. sp hybridised carbon 15 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.
An sp hybridised carbon atom
sp hybrid
C2p y z

35. Triple bonding in Ethyne16
Triple bonding in Ethyne
Two sp hybridised carbons can form a covalentbond.
Other hybrid orbitals covalently bond to two hydrogens.
C2p z
C2p z
C2p y
C2p y

36. Pi () bonding in Ethyne16
Pi () bonding in Ethyne
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.
Two sets of clouds (shown in green and blue) form y and z bonding orbital. C H y z
C2p z CH s
bond CC y

37. Ethyne (acetylene) 16
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.

38. Bond length—strength CC bonds17
Bond length—strength CC bonds
Summary: pm
Bond length decreases from single to double to triple bond.
Bond strength increases from single to double to triple bond.

39. Functional Groups Alcohols18
Functional Groups Alcohols
CH3OH
Methanol

40. Functional Groups Alcohols18
Functional Groups Alcohols R OH
R = alkyl group,
OH = hydroxyl group ..
CH3CH2OH
Ethanol

41. Functional Groups Alcohols18
Functional Groups Alcohols
Classification:

42. Functional Groups Amines19
Functional Groups Amines
In methylamine, sp3 nitrogen is covalently bonded to methyl and two hydrogens N CH 3 H C
Methylamine (Methanamine)

43. Functional Groups Amines19
Functional Groups Amines
Classified on number of alkyl groups attached to nitrogen

44. Functional Groups Ketones and Aldehydes20
Functional Groups Ketones and Aldehydes R
CHO
R = organic group,
CHO = aldehyde group C H O
R = organic groups,
CO = ketonic group C R O CO

45. Functional Groups Ketones and Aldehydes20
Functional Groups Ketones 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: :
O2p z
C2p CO s
-bond H C O H
Polarised p
-molecular
orbital
120°

46. Functional Groups Ketones and Aldehydes20
Carbon is positively polarised and oxygen negatively polarised
Carbonyls are best seen as:

47. Functional Groups Carboxylic acids21
Functional Groups Carboxylic acids R
CO2H
R = alkyl group,
CO2H = carboxyl group

48. Functional Groups Carboxylic acids21
Functional Groups Carboxylic acids
Why acidic?
In water they ionize partially

49. Functional Groups Carboxylic acids21
Functional Groups Carboxylic acids
Resonance:
Negative charge is on both oxygens