GRADE 12 SCIENCE
ORGANIC CHEMISTRY
Organic chemistry is the study of the compounds of carbon, but does not include carbonates hydrogen
carbonates, carbides and the oxides of carbon.
There are millions of organic compounds; some very simple and others complicated with large molecular
masses. The reasons for the large variety of compounds are that carbon atoms:
1.
can covalently bond with one another to form straight or branched chains (catenation).
2.
can covalently bond with other atoms - usually non-metals.
3.
can supply four electrons for four different covalent bonds.
4.
Can form double, single or triple bonds
Hydrocarbons – contain carbon and hydrogen ONLY
Formulae of Organic Compounds
 Molecular formula – no of atoms of each element preset
 Structural formula – shows all the atoms and bonds in the molecule
 Condensed structural formula does not show C-C and C-H single bonds
Molecular formula
Structural formula
Condensed structural formula
H
H H H
H C C C C C C H
H H H H H H
H
H
H
H H C H H
H C C C C C H
H H H OH H
Functional Groups and Homologous Series
Definition: A homologous series is a family of organic compounds having the same functional group but
different chain lengths.
A functional group is the part of the molecule that determines its chemistry.
Properties of homologous series:
1. They have the same general formula (e.g. alkanes CnH2n+2).
2. Each member differs from the next by -CH2-. (e.g alkanes CH4, C2H6, C3H8…)
3. They have similar chemical properties. (e.g all alkanes burn to form CO2 and H2O)
4. Their physical properties change gradually as their molecular masses increase. The higher the mass
the greater the intermolecular (van der Waal) forces. This results in increasing melting and boiling
points, increased viscosity and decreasing vapour pressure. (e.g. methane (CH4) is a gas but pentane
(C5H12) is a liquid)
5. They can be prepared by the same general methods.
IUPAC naming of organic compounds
The name has three parts
Prefix
Where and what
other groups
1.
2.
3.
Parent
Number of
carbon atoms
Suffix
Homologous
series
Identify the longest chain containing the main functional group (homologous series).
Number carbons in the chain and give it the parent name.
Identify functional groups and side chains.
The prefix for the length of a chain (or side chain) is taken from the following:
1C- meth
2C-
eth
3C-
prop
4C- but
5C- pent
6C-
hex
7C-
hept
8C- oct
SERIES
FUNCTIONAL
GROUP
ALKANE
C-C
ETHANE
C2H6
ALKENE
C=C
PROPENE
C3H6
ALKYNE
C≡C
ETHYNE
C2H2
ALCOHOL
C-OH
METHANOL
CH3OH
O
ALDEHYDE
C
C
H
O
KETONE
CARBOXYLIC
ACID
C
C
C
O
C
O H
EXAMPLE
Structural formula
NAME
(formula)
PROPANAL
C2H5CHO
PROPANONE
CH3COCH3
ETHANOIC ACID
CH3COOH
HALOALKANE
-Cl....chloro-Br....bromo-I.......iodo-
2-BROMOPROPANE
CH3CHBrCH3
ESTER
O
C C
O C
METHYL ETHANOATE
CH3COOCH3
NOTE: There are 4 bonds per carbon (ALWAYS).
The position of the side chain or functional group is shown with a number. The choice of number
is usually such that the smallest possible numbers are used.
The -an- in compounds (e.g. butanol) tells one that the C to C bonds are single bonds only.
Name the following compounds:
H
H H H O
H C C C C H
H O H C
H
H H
H
2.
H H
H H H
H C C C C C C C H
H H
H H H
1.
H
O
O
H3C CH2 CH2 C
H3C CH2 CH2 C CH2 CH2 C
CH2
H
H3C
3.
4.
5. CH2CHCHCHBrCH2CH3
6. CH3CH2COCH2CH2CH2CH3
O CH2 CH3
Complete the following table:
Compound name
hept-3-yne
4-bromo-2,4-dimethylhex-1-ene
3-chloropent-2-one
5,7-dimethyloctanal
2-iodohexanoic acid
butylpentanoate
Structural formula
Condensed structural formula
ISOMERS
Definition: molecules with the same molecular formula but different structures.
There are several types of isomerism - we only deal with STRUCTURAL ISOMERISM which has 3 sub-types:
CHAIN ISOMERISM
different chains
Eg C4H10
Note:
POSITIONAL ISOMERISM
functional groups at
different positions on
chain eg C5H11OH
FUNCTIONAL ISOMERISM
different functional
groups eg C3H6O
Sometimes the isomer is in a different homologous series.
Isomers will have different physical properties and chemical properties.
Exercise: Draw the structural formula and name all the isomer bromoalkanes with the formula:
C4H9Br
THE ALKANES (suffix ‘-ane’)
- hydrocarbons with the general formula CnH2n+2.(Note: a cycloalkane will have the formula CnH2n.)
- have only C - C single bonds and are said to be saturated
e.g.
2,2-dimethylpropane
H
H
C H H
H C C C H
H
C H H
H
H
H
We draw them in 2-D but they are actually 3-D and the carbon chain zig-zags.
REACTIONS OF ALKANES:
- generally unreactive
COMBUSTION:
- an oxidation reaction
- burn in a plentiful supply of air to produce carbon dioxide and water
- exothermic reaction.
- in a limited air supply incomplete combustion occurs and carbon monoxide is also obtained.
alkane + O2  H2O + CO2
∆H<0
eg octane
HALOGENATION:
- a substitution reaction (one or more hydrogens are replaced by a functional group).
- reaction conditions: X2 (X = Br, Cℓ) added to the alkane in the presence of light or heat
- the halogen will substitute the hydrogen very slowly in the dark. In light the reaction is speeded up.
CH4 + Cl2  CH3Cl + HCl
eg.
- halogenation may continue until all the hydrogens have been replaced so that a reaction between
methane and chlorine will produce a mixture of CH3Cl and:
CH2Cl2
CHCl3
CCl4
dichloromethane
trichloromethane (chloroform)
tetrachloromethane
CRACKING:
- Breaking longer chain alkanes that have limited uses into smaller and more useful bits
- an elimination reaction (saturated compounds eliminate atoms to become unsaturated).
- reaction conditions: Thermal cracking - high temperatures (450°C to 750°C) and pressures (up to 70 atm)
- achieves high proportions of hydrocarbons with double bonds – alkenes
Catalytic cracking - catalyst (zeolites), moderate temperature and pressure
- achieves high percentage of chains with between 5 and 10 carbon atoms
- hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller
hydrocarbons, some of which have carbon-carbon double bonds.
eg.
C15H32  2C2H4 + C3H6 + C8H18
OR
C15H32  C2H4 + C4H8 + C9H20
ALKANES and INDUSTRY
- Alkanes are the main constituent of crude oil.
- Crude oil is largely a mixture of alkanes of varying size and shape.
- Due to their varying boiling points crude oil can be separated into fractions by distillation.
- The fractions have various industrial uses, but because the combustion of alkanes is so exothermic, the
most important use is as a source of energy. Petrol is one such fraction.
- The longer chain hydrocarbons in crude oil have limited applications whilst shorter ones are in high
demand
- Cracking provides the shorter alkanes used as fuels and small alkenes that are used in polymer synthesis
THE ALKENES (suffix ‘-ene’)
- hydrocarbons with the general formula CnH2n.
- contain at least one C to C double bond and are said to be unsaturated.
- if an alkene has two double bonds it is a diene.
- formed by cracking alkanes or elimination (water is eliminated) reactions of alcohols (see later)
REACTIONS OF ALKENES
- more reactive than alkanes due to unsaturation
COMBUSTION: as for alkanes if there is enough oxygen.
eg. butene and oxygen
HALOGENATION:
- an addition reaction (a reagent adds ACROSS the double bond, breaking one of the bonds
(weaker π-bond), resulting in a saturated product.
- addition of X2 (X = Cℓ, Br) across the double bond of alkenes to give a dihaloalkane
- reaction conditions: X2 (X = Br, Cℓ) added to alkene
eg.
CH2CH2 + Br2  CH2BrCH2Br
- used as a test for an unsaturated compounds as the brown bromine is rapidly decolourised by
alkenes
HYDROHALOGENATION:
- addition of HX (X = Cℓ, Br, I) to an alkene to give a haloalkane
- reaction conditions: HX (X = Cℓ, Br, I) added to alkene; no water must be present
eg.
CH2CH2 + HBr  CH3CH2Br
- if the alkene is not symmetrical, the H atom attaches to the C atom already having the greater
number of H atoms
eg.
CH3CHCH2 + HI 
HYDRATION:
- Addition of H2O across the double bond to give an alcohol
- reaction conditions: H2O in excess and a small amount of strong acid (often H3PO4) as catalyst,
high temperature (ie H2O is steam)
eg.
CH2CH2 + H2O  CH3CH2OH
- if the alkene is not symmetrical, the H atom attaches to the C atom already having the greater
number of H atoms.
eg.
CH3CHCHCH2CH3 + H2O 
HYDROGENATION:
- Addition of H2 across the double bond to give an alkane
- reaction conditions: alkene dissolved in a non polar solvent with catalyst (Pt, Pd or Ni),
high pressure H2 atmosphere
eg.
CH2CH2 + H2  CH3 CH3
- important reaction in food industry as hydrogenation converts liquid vegetable oils into solid or
semi-solid fats, such as those in margarine
THE ALKYNES (suffix ‘-yne’)
- hydrocarbons with the general formula CnH2n-2
eg.
ethyne
but-2-yne
1-chlorohept-3-yne
REACTIONS OF ALKYNES
- reactive due to unsaturation – undergoes similar addition reactions to the alkenes
COMBUSTION:
- Ethyne (acetylene) is used in oxy-acetylene torches. It burns at a very high temperature in a high
concentration of oxygen.
2C2H2 + 5O2  4CO2 + 2H2O
THE ALCOHOLS (suffix ‘-ol’)
- The functional group is the hydroxyl(–O-H)
- they may have one or more OH groups (-diol, -triol etc)
eg. propan-1,2,3-triol (glycerol - used in soaps and medicines)
- Mono-alcohols have the general formula CnH2n+1O-H.
Primary Alcohols
Secondary Alcohols
The O-H group is attached to a
The O-H group is attached to a
carbon to which one other
carbon to which two other
carbon is attached.
carbons are attached.
eg. 1-butanol
eg. 3-pentanol
Tertiary Alcohols
The O-H group is attached to a
carbon to which three other
carbons are attached
eg. 2-methyl-2-butanol
- industrial preparation - reaction between steam and alkenes (addition/hydration – see alkenes)
Note – ethanol is the ONLY primary alcohol that can be prepared like this.
REACTIONS OF ALCOHOLS
COMBUSTION - as for alkanes
eg. burning ethanol
HALOGENATION:
- a substitution reaction (OH group is replaced by a halogen).
- the substitution proceeds readily for tertiary alcohols, but is slow and needs heating for primary
and secondary alcohols
- reaction conditions:
Primary and Secondary Alcohols
- treat with concentrated H2SO4 and solid NaBr (or KBr)
- H2SO4 and NaBr react to form HBr
H2SO4 + NaBr  HBr + NaHSO4
- HBr reacts with the alcohol to form the bromoalkane
- Overall reaction eg.
CH3CH2CH2OH + H2SO4 + NaBr
Tertiary Alcohols
- use HBr or HCℓ at room temperature
eg.
C(CH3)3OH + HBr
C(CH3)3Br + H2O
CH3CH2CH2Br + H2O + NaHSO4
DEHYDRATION:
- an elimination reaction (saturated compounds eliminate atoms to become unsaturated).
- in alcohols water is eliminated to form an alkene
- reaction is acid catalysed
- reaction conditions: heating of alcohol with an excess of concentrated H2SO4 (or H3PO4).
eg.CH3CH2OH  CH2=CH2 + H2O
- If more than one elimination product is possible, the major product is the one where the
H atom is removed from the C atom with the least number of H atoms
eg. 2-butanol will react to form mainly 2-butene (not 1-butene).
THE HALOALKANES (prefix ‘halo-’) – also called ALKYL HALIDES
- contain 1 or more halogen atoms: (Cl  chloro-; Br  bromo-; I  iodo-)
- they may have one or more OH groups (-diol, -triol etc)
- as for alcohols, primary secondary and tertiary halides, depending on the degree of branching in
the carbon to which the halide is attached
eg. 1-bromopropane
2-iodopropanoic acid
3-bromo-3-methylpentane
REACTIONS OF HALOALKANES
HYDROLYSIS:
- a substitution reaction
- aqueous bases react with haloalkanes to produce alcohols
- reaction conditions:- haloalkane dissolved in ethanol, aqueous NaOH or KOH (dilute solution)
added and mixture warmed gently
- (same reaction occurs without NaOH, but more slowly)
eg.
CH3CHBrCH3 + NaOH  CH3CH2OHCH3 + NaBr
DEHYDROHALOGENATION:
- an elimination reaction
- elimination of HX from a haloalkane to produce an alkene
- reaction conditions:- heat under reflux in a concentrated solution of
NaOH or KOH in pure ethanol
- (NO H2O present)
eg.
CH3CHClCH3 + NaOH  CH3CH=CH2 + NaCl + water
- If more than one elimination product is possible, the major product is
the one where the H atom is removed from the C atom with the
least number of H atoms
e.g. reaction of 2 – bromobutane with hot, ethanolic KOH:
THE CARBOXYLIC ACIDS (suffix ‘oic acid’)
- functional group is carboxyl (-COOH).
- COOH can only be found at the end of a chain and the carbon it is on is number 1.
- carboxylic acids are weak acids – low Ka values, partially ionize in water
e.g.
2-iodobutanoic acid
REFLUX
solvent vapours
condense and
return to flask
during heating)
REACTIONS OF CARBOXYLIC ACIDS
ESTERIFICATION:
- a condensation reaction (two larger molecules are joined by the elimination of a smaller molecule)
- an ester and water are formed in the reaction
- reaction conditions:- carboxylic acid is mixed with an alcohol
- a little H2SO4 is added as a catalyst
- mixture warmed gently
eg.
CH3CH2COOH + CH3OH  CH3CH2COOCH3 + H2O
THE ESTERS (name ‘-yl -oate’)
- formed by the reaction of an alcohol and a carboxylic acid, from which the name is derived.
-yl
-oate
eg. Ester from the reaction between butanol and hexanoic acid is named butyl hexanoate
ESTERS and INDUSTRY
- widely found in nature – simple ones are often are sweet smelling and give fruit and flowers their
characteristic flavours and fragrances. Food and beverage manufacturers often add them to
products to enhance odour and flavor of products
- smaller esters have low boiling points frequently used as solvents where fast evaporation is needed eg.
glue manufacture
THE KETONES (suffix ‘-one’)
- contain the carbonyl (C=O) functional group,
- carbonyl carbon is bonded to two other carbons
R
O
C
R
1
R and R1 are alkyl chains eg CH3 or CH2CH3
- condensed formula -CO- eg. propanone CH3COCH3
- ketones are polar solvents with low boiling points
eg. propanone (common name acetone) – main ingredient in nail polish remover
THE ALDEHYDES (suffix ‘-al’)
- contain the carbonyl (C=O) functional group, always at the END of a carbon chain
- carbonyl carbon is bonded to ONE other carbon and ONE hydrogen
R
O
C
R is an alkyl chains eg CH3 or CH2CH3
H
- condensed formula -CHO eg. propanal CH3CH2OCHO
- more reactive than ketones … thus frequently unstable