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 sub- orbitals are known as sp3 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 C C H H H H
Saturated compound – compounds in which all bonds between the carbon atoms are single bonds. H H Alkenes H C C H
Unsaturated compound – compounds in which there is at least one double and/or triple bond between carbon atoms. Homologous Series and Functional groups
Alkanes CnH2n+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 C H C2H6 C3H8 4 10
Alkenes CnH2n H H H H H H H H H H C C C C H H C C H H C C C H C H H H C2H4 3 6 H C4H8 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
C4H10 C4H8 Organic nomenclature Straight chained hydrocarbon molecules The functional group determines the suffix of the name: Alkanes (single bonds) end in -ane. Alkenes (double bond) end in -ene.
ethene ethane
Branched hydrocarbon molecules 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. 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.
CH3 2- methylpenta -1,3 - diene 2 – methylhexa – 1,4 - diene
CH2 CH2
2-methylpent-1-ene Haloalkanes (Alkyl halides) General formula : CC nH2n+1X 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 : CnH2n+1OH or CnH2n+2O
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 CnH2nO2
Esters : CnH2nO2 Ester formation Isomerism Isomers – organic molecules which have the same molecular formula but different structural formulae. C6H14
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
CH2 = CH – CH2 –CH2 – CH3 CH3 – CH = CH – CH2 – CH3 C5H10 pent – 1 – ene pent – 2 – ene • Functional isomers – isomers that contain different functional groups (eg) carboxylic acids and esters O O
CH3 – CH2 – CH2 – CH2 – C – OH C 5 H 10 O 2 CH3 – CH2 – O – C – CH2 – CH3 pentanoic acid ethyl propanoate
Summary of Isomerism Exercise
1. 2. 3.
4. 5. 6.
CH2 – CH – CH2
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
C3H8 + 5O2 3CO2 + 4H2O Butane burns in oxygen
C4H10 + 6½O2 4CO2 + 5H2O
(x 2) 2C4H10 + 13O2 8CO2 + 10H2O Ethanol burns in oxygen
C2H6O + 3O2 2CO2 + 3H2O
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 Halogenation
hydrohalide (HCl) water (H2O)
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 + H2O H H H H H H • Cracking - the breaking up of large hydrocarbon molecules into smaller, more useful molecules
and Past examination Question Memorandum