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Week 5 - ORGANIC CHEMISTRY (2) - Updated

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ORGANIC CHEMISTRY

Organic chemistry is often described as the chemistry of carbon-based

compounds that consist primarily of carbon and hydrogen.

The unique chemistry of carbon

• Carbon atoms have the ability to form four strong covalent bonds

• Carbon undergoes a process known as hybridization which produces four available bonding

sites ( see “process of hybridization”)

• Carbon atoms bond with other carbon atoms to form chains or ring structures. This is called

catenation These chains can be thousands of atoms long.

• Carbon has the ability to make single, double and triple bonds with itself

Catenation is described as the ability of carbon atoms to bond with themselves to form chain or

ring structures


The process of hybridisation

A carbon atom in the ground state:

C

A carbon atom in the “excited” state:

4 x sp3 hyhrid sub-orbitals

able to accept one electron

each

A process called orbital mixing now occurs where the 2s and 2p orbital’s

now mix together to produce four sub-orbitals of equal energy. There suborbitals

are known as sp 3 hybrid orbital’s and it is these hybrid orbital’s

that provide the four available bonding sites


Classification of organic compounds


THE HYDROCARBONS……are organic compounds containing carbon and

hydrogen only

H

H H

Alkanes

H

C

H

H

Saturated compound – compounds in which all bonds between the carbon atoms

are single bonds.

Alkenes

H

H

C

H

C

H

Unsaturated compound – compounds in which there is at least one double and/or

triple bond between carbon atoms.

H

C

H

C

H

H


Homologous Series and Functional groups

Alkanes C n H 2n+2

H

H

H

H

H

H

H

H

H

H

C

C

H

H

C

C

C H H C

C

C

C

H

H

H

H

H

H

H

H

H

H

Alkenes

C 2 H 6 C 3 H 8

C 4 H 10

C n H 2n

H H

H C C

H H H

H H

H

H C C C H

H C C

C 2 H C 4 3 H 6 H

C 4 H 8

H

C

H

H

C

H

H


Homologous Series and Functional Groups

Functional group - a bond, atom or group of atoms that form the centre of chemical

activity in the organic compound. ( also identifies to which Homologous Series an organic

compound belongs

carbon – carbon

single bond

or…..halo group


Representing organic compounds

C 4 H 10 C 4 H 8


Organic nomenclature

Straight chained hydrocarbon molecules

The functional group determines the suffix of the

name:

Alkanes (single bonds) end in

Alkenes (double bond) end in

-ane.

-ene.

ethane

ethene

Numerous organic molecules are found to have carbon based side chains

attached to a main chain within the structure. These side chains are simply

known as side branches or more scientifically correct – Alkyl Substituents

Alkyl substituent – a carbon based “side chain” which

is attached to the longest continuous carbon chain in an

organic molecule.

Branched hydrocarbon molecules


Naming branched chained alkanes


Naming unsaturated hydrocarbons (alkenes)


Exercise 1 : Name the following hydrocarbons

1. 2.

3.

4.

5. 6.


Memorandum

1. 2-methylbutane

2. 2,3-dimethylbutane

3. 3-ethyl-3-methylpentane

4. 3-methylbut-1-ene

5. 3,3-dimethylbut-1-ene

6. 6-methylhept-3-ene


Hydrocarbons with more than one double bond in the chain (dienes)

The rules work exactly the same in all nomenclature, the only difference is that

now two positions must be stated in the naming. These positions must still be at

the lowest substituted position in the molecule

For euphonic purposes, the vowel "a" must be added to the name before

the suffix for dienes, e.g. buta−1,3−diene is correct, and not but−1,3−diene.

2- methylpenta -1,3 - diene 2 – methylhexa – 1,4 - diene

CH 3

CH 2 CH 2

2-methylpent-1-ene


Haloalkanes (Alkyl halides) General formula : C n H 2n+1 X

Functional group = X where X = F, Cl, Br, I ( halo functional group)

The position of the halogen is specified by the lowest substituted carbon to which that halogen is

attached.

Naming : The haloalkane is named with the lowest substituted carbon having the halogen

attached placed first in the naming sequence

3


Alcohols: General formula : C n H 2n+1 OH or C n H 2n+2 O

Ethanol is used widely as a solvent in paints, glues, perfumes,

aftershaves and any other household products.

The strong hydrogen bonds in alcohols result in alcohols having

higher melting and boiling points than hydrocarbons of similar

size.

Functional group:

OH

Pentane -2,3 - diol


Carboxylic Acid : General formula C n H 2n O 2


Esters : C n H 2n O 2


Ester formation


Isomerism

Isomers – organic molecules which have the same molecular formula but different structural

formulae.

C 6 H 14

2,2 – dimethylbutane 2 – methylpentane

There are various other types of structural isomers that can be found in organic chemistry

• Chain isomers – these are isomers that will have different chain lengths. The examples seen above

are those of chain isomers

• Positional isomers – these are isomers that have a different position of the same functional group

CH 2 = CH – CH 2 –CH 2 – CH 3 CH 3 – CH = CH – CH 2 – CH

C 3

5 H 10

pent – 1 – ene

pent – 2 – ene

• Functional isomers – isomers that contain different functional groups (eg) carboxylic acids and esters

O

O

C 5 H 10 O 2

CH 3 – CH 2 – CH 2 – CH 2 – C – OH CH 3 – CH 2 – O – C – CH 2 – CH 3

pentanoic acid

ethyl propanoate


Summary of Isomerism


Exercise

1. 2. 3.

4. 5. 6.

CH 2 – CH – CH 2

O-H

O-H

O-H

7.


Memorandum

1. 1,2-dichloropropane

2. 1,1-dichloro-2,2-difluoroethane

3. Butan-2-ol

4. 2-methylbutan-2-ol

5. Propane-1,2,3-triol

6. Butyl propanoate

7. 3-methylpropa-1,2-diene


Practice Example 2


Memorandum 2


Organic Chemical Reactions

Organic chemical reactions can be classified into FIVE different types of reactions based on

how the molecule is able to react under certain reaction conditions

• Combustion • Addition

• Substitution • Elimination

• Esterification - seen already

Combustion

Alkanes, alkenes and alcohols burn in oxygen and form carbon dioxide and water.

The reaction is exothermic and a great deal of energy is released.

Propane is the gas used in Bunsen burners in the laboratory

C 3 H 8 + 5O 2 3CO 2 + 4H 2 O

Butane burns in oxygen

C 4 H 10 + 6½O 2 4CO 2 + 5H 2 O

(x 2) 2C 4 H 10 + 13O 2 8CO 2 + 10H 2 O

Ethanol burns in oxygen

C 2 H 6 O + 3O 2 2CO 2 + 3H 2 O


Addition - when a double bond is broken and new molecular fragments are added to both

ends of the bond

In the double bond, one of the bonds is very weak

and will break under reaction conditions to form two

unpaired electrons on each carbon. These radicals are

highly reactive and will this bond to form an electron

pair

Hydrogenation

hydrohalide (HCl)

Halogenation

water (H 2 O)

Hydrohalogenation

Hydration


Substitution - “swapping” reactions

• Halogenation (Free radical substitution) – alkanes to haloalkanes

• Hydrolysis – haloalkanes to alcohols


Elimination This is the opposite of addition where functional groups are removed to form an

alkene

• Dehydrohalogenation

H H Cl H

H - C - C - C - H H - C - C = C - H + HCl

H H H H H H

• Dehydration

H H OH H

H - C - C - C - H H - C - C = C - H + H 2 O

H H H H H H

• Cracking - the breaking up of large hydrocarbon molecules into smaller, more useful molecules

and


Past examination Question


Memorandum

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