Chap 10B - Organic chemistry

Describe purpose of hybridisation

• If carbon were to use its 2s and 2p orbitals for bond formation, many structural features of carbon compounds cannot be accounted for

• For example, in CH4, the bond angle would be 90° instead of the actual value of 109.5° and the four C-H bonds 

• Theory of hybridisation (mixing of orbitals) was proposed to address these discrepancies

Describe characteristics of hybridised orbitals

1. Each sp, sp2 or sp3 hybrid orbital has one lobe larger than the other

• In bond formation, the larger lobe is used -> more effective overlapping -> stronger bonds than if the unhybridised 2s or 2p orbitals had been used

2. The more s character a hybrid orbital has, the closer the electrons are to the nucleus

• More s character -> stronger bond 

• There is greater s-character in the sp2 hybrid orbital than in the sp3 hybrid orbital and the electrons in the sp2 hybrid orbital are closer to the nucleus -> C-C bond with sp2-sp2 overlap is shorter than that with sp3-sp2 overlap

Describe sp³ hybridisation

• Four orbitals of carbon (2s, 2px, 2py, 2pz) are 'mixed' to form four equivalent sp3 hybrid orbitals which are tetrahedral and at 109.5° to one another

• The hybrid orbitals have the same energy

• The carbon atom uses the four sp³ hybrid orbitals to form 4 sigma bonds (all single bonds) with other atoms

• Each sp hybrid orbital contains ¼ s and ¾ p character

• Eg. Trigonal pyramidal: 4 regions of electron densities (3 sigma, 1 lone) = sp3 hybridised

Describe sp² hybridisation

• Three orbitals of carbon (2s, 2px, 2py) are 'mixed' to form three equivalent sp² hybrid orbitals which are trigonal planar and at 120° to one another.

• One 2pz orbital of carbon remains unchanged

• The carbon atom uses the three sp² hybrid orbitals to form 3 sigma bonds and the unhybridised 2p orbital to form 1 pi bond (1 double bond present) with other atoms

• Each sp² hybrid orbital contains ⅓s and ⅔ p character

Describe sp hybridisation

• 2 orbitals of carbon (2s, 2px) are ‘mixed’ to form 2 equivalent sp hybrid orbitals which are linear and 180 degrees to each other 

• 2 2p orbitals of carbon remain unchanged 

• Carbon atom uses 2 sp orbitals to form 2 sigma bonds and 2 unhybridised 2p orbitals to form 2 pi bonds (triple bond or 2 double bonds) with other atoms 

• Each sp hybrid orbital contains ½ s and ½ p characters

Describe hybridisation in CH4

Carbon atoms forms four sigma bonds: 

1. Number of hybrid orbitals: 4

2. Type of hybridisation: sp3

3. Shape: tetrahedral around carbon atom, bond angle = 109.5°

4. Sigma bond formation: each C-H bond is formed by the head-on overlap of sp3 hybrid orbital of C with 1s orbital of H

Describe hybridisation in C2H6 ethane

Each carbon atom forms four sigma bonds: 

1. Number of hybrid orbitals: 4

2. Type of hybridisation: sp3

3. Shape: tetrahedral around each carbon atom, bond angle = 109.5°

4. Sigma bond formation: each C-H bond is formed by the head-on overlap of sp3 hybrid orbital of C with 1s orbital of H and the C-C bond is formed by the head-on overlap of sp3 hybrid orbital of each C

Describe hybridisation in ethene C2H4

Each carbon atom forms three sigma bonds and one pi bond: 

1. Number of hybrid orbitals: 3

2. Number of p orbitals: 1

3. Type of hybridisation: sp2

4. Shape: trigonal planar around each carbon atom, bond angle = 120°

5. Sigma bond formation: each C-H bond is formed by the head-on overlap of sp2 hybrid orbital of C with 1st of H and the C-C bond is formed by head-on overlap of sp2 hybrid orbital of each C

6. Pi bond formation: carbon-carbon pi bond is formed by the side-on overlap of 2pz orbital of each C laterally -> resulting pi electron cloud is distributed above and below the plane or atoms

Describe hybridisation in Benzene, C6H6

Each carbon atom forms three sigma bonds and one pi bond: 

1. Number of hybrid orbitals: 3

2. Number of p orbitals: 1

3. Type of hybridisation: sp2

4. Shape: trigonal planar around each carbon atom, bond angle = 120°

Hence, benzene has a planar structure in which the six carbon atoms are bonded in a regular hexagonal ring

5. Sigma bond formation: each C-H bond is formed by head-on overlap of sp2 hybrid orbital of C with 1s orbital of H and the C-C bond is formed by the head-on overlap of sp2 hybrid orbital of each C 

6. Pi bond formation: C-C pi bond is formed by the side-on overlap of 2px orbital of each C with the adjacent 2pz orbitals on either side of it

Describe electron cloud in benzene

• Since these six 2p orbitals are parallel to one another, each of the 2p orbital can overlap sideways and equally with the adjacent two 2p orbitals

• Each of the six 2p unhybridised orbital contains an electron

• Hence, the sideways overlap of the 2p orbitals results in a pielectron cloud above and below the hexagonal plane of carbon atoms

NOTE: 

1. The pi electrons move over a larger region of space in the actual structure benzene, as compared to the Kekule structure

• The pi electrons are delocalised around the whole ring (resonance)

2. Each C-C bond in benzene is a partial double bond and is identical to one another

2. Structure of benzene is more accurately presented as (somewhere above) where the circle represents the delocalisation of six pi electrons

Describe hybridisation in Ethyne, C2H2

Each carbon atom forms two sigma bonds and two pi bonds: 

1. Number of hybrid orbitals: 2

2. Number of p orbitals: 2

3. Type of hybridisation: sp

4. Shape: linear around each carbon atom, bond angle = 180°

5. Sigma bond formation: each C-H bond is formed by the head-on overlap of sp hybrid orbital of C with 1s orbital of H and the C-C bond is formed by the head-on overlap of sp hybrid orbital of each C

6. Pi bond formation: one carbon-carbon pi bond is formed by the side-on overlap of 2py orbital of each C laterally and the other carbon-carbon pi bond is formed by the side-on overlap of 2pz orbital of each C laterally

• Pi electron cloud forms a cylindrical sheath around the axis joining the carbon atoms

Define substituent

• A substituent (usually alkyl or aryl) refers to an atom or group of atoms that takes the place of a hydrogen atom on the hydrocarbon chain

Describe alkyl group

• Is a type of substituent derived from alkanes by removing a hydrogen atom

• Group of carbon and hydrogen having general formula CnH2n+1 

• Denoted by letter R 

• Name: follows corresponding alkane but with -yl 

• Eg. Methyl , Ethyl 

Describe aryl group

• Is a type of substituent derived from an aromatic ring (such as a benzene ring) by removing a hydrogen atom

• Some organic compounds are classified based on how many substituents are joined to the carbon atom bearing the functional group

Draw hybridisation of ethane

Draw hybridisation ethene

Draw hybridisation of benzene

Draw hybridisation ethyne