Alkanes

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Alkanes. These are hydrocarbons which form a homologous series of saturated nature with a general formula
CnH2n+2. Successive members differ in composition by a methylene group.

The main sources of Alkanes include:

• Natural gas which mainly contains methane with low amounts or propane and butane
• Petroleum products which are separated by fractional distillation during a process of refining

Cracking
This is the breaking down of long chain organic molecules (hydrocarbon) into two or small molecules
which are useful products.
Cracking can be done using a catalyst (catalytic cracking) or by using high temperature (thermal cracking), the alkenes are used to make other chemical and alkanes are used for fuels.

NOMENCLATURE/NAMING OF ALKANES
With exception of the first four members of the homologous series, the straight chain members are named by taking prefix appropriate to the number of the carbons and adding the ending –ane. For branched members, the longest chain is selected and prefix is added to the name of the main of the chain, the position of the substituent is also indicated by appropriate number
Examples
Name the following organic compounds according IUPAC system:

Solution
(a) Butane (b) Pentane (c) Heptane (d) 2-methylbutane
(e) 2, 2-dimethylpropane (f) Cyclobutane

ISOMERISM IN ALKANES

Alkane’s posse’s structural isomers of class chain isomers from Butane.
Example
A hydrocarbon Q has a vapour density of 36 with empirical formula C5H12.
(a) Determine the molecular formula of Q
(b) Write down the possible isomers of Q and their IUPAC names
Solution:
(a) Molecular mass = 36×2 = 72g

PREPARATION OF ALKANES

1. HYDROGENATION OF ALKENES AND ALKYNES
   This method provides one of the most suitable methods for preparing Alkanes. It used to deduce the number of the double or triple bond in the unsaturated hydrocarbon.
   The method requires catalyst such as Nickel, Platinum or Palladium. Nickel requires an elevated temperature of 150oC whereas Platinum and Palladium requires ordinary temperature.
   This reaction is used to determine the molecular mass of the Alkanes or Alkenes or Alkynes based upon the stoichiometry of the reaction

Example
2.24dm3 of Hydrogen gas at s.t.p reacts with 8.4g of hydrocarbon Q as represented by the equation below:

(a) Identify the functional group in the hydrocarbon Q
(b) Determine the molecular mass of Q hence that of R
(c) Write the:
(i) Molecular formula of Q
(ii) Possible isomers of Q and their IUPAC names
(1 mole of a gas occupies 22.4dm3)

Solution:
(a) Alkenes

(b) Moles of Hydrogen gas reacted

Moles of the Alkenes = 0.1 (since the mole ratio is 1: 1) hence the RFM = 0.1×8.4 = 84
Molecular mass of Q = 84g
R + 2(12) + 3(1) = 84 thus R = 57g

2. KOLBES SYNTHESIS
   When aqueous solution of sodium salt of the carboxylic acid is electrolysed, an alkane is liberated at the anode and carbondioxide at the cathode. This suitable for preparing symmetrical alkane of the R-R .

3. DECARBOXYLATION METHOD
   The sodium salt of the carboxylic acid is heated with soda lime which contains sodium hydroxide and
   calcium oxide. This reaction decreases the carbon chain by one carbon

4. FROM ALKYL HALIDES
   Reduction of alkyl halides also generates Alkanes. This can be done by using Zinc-Copper couple in
   ethanol or using lithium aluminium hydride in dry ether.

Hydrolysis of Grignard reagent also gives rise to alkane

5. WURTZ REACTION
   Haloalkanes react with metallic sodium in dry ether to yield Alkanes of higher molecular mass and the
   carbons atoms are in even numbers. Greater yield is obtained when the alkane has a higher molecular mass.

FOR METHANE
Hydrolysis of Aluminium carbide with hot water or dilute mineral acids form methane

N.B. The diagonal carbide can also be used i.e. Beryllium carbide
PHYSICAL PROPERTIES OF ALKANES
The straight chainsAlkanes, C1-C4 are gases, C5-C7 are liquids and the remaining homologues are solids.
Boiling and Melting points increase with increasing molecular mass although the branched isomers are more volatile and degree of volatility increases with branching.

Alkanes are immiscible with polar solvents such as water but soluble in non-polar solvents such as chloroform, ether etc.
Density of the Alkanes increase with molecular mass but the factor reduces with increase in the branching.
REACTIONS OF ALKANES
Alkanes are generally unreactive compared to alkenes and alkynes.
Reasons:
– The C-C and C-H bonds are strong therefore unless heated to higher temperatures or subjected to ultraviolet light.
– Carbon and Hydrogen have nearly the same electronegativity value therefore the bonds are slightly polarized.
– There are no lone pairs of electrons in Alkanes therefore Alkanes are not attacked by acids. However, under certain conditions, some of the reactions of Alkanes (paraffins) include:

1. COMBUSTION
   Alkane’s burn in excess oxygen to form carbondioxide and water,the reaction is exothermic and it can be used to determine the molecular formula of the Alkanes.

2. CHLORINATION OR BROMINATION
   Alkanes react with halogens when heated at 250-400oC or in presence of Ultraviolet light to form a
   mixture of products which are difficult to separate.
   The reactivity is the order of Fluorine> Chlorine> Bromine> Iodine. Iodine does not react with
   Alkanes while Fluorine reacts explosively.

Mechanism

N.B. The reaction mechanisms repeat to generate the rest of the products.

3. NITRATION REACTION
   Alkanes react with nitric acid at about 400oC via free radical mechanism to form nitroalkane

For example

HALOGENATION USING SULPHURYL CHLORIDE

ALKENES (OLEFINS)

Alkenes form a homologous series of unsaturated hydrocarbons containing a carbon-carbon double
bond with the general formula CnH2n .
NOMENCLATURE OF ALKENES
In accordance with the IUPAC system, an alkene is named by dropping the ending ane from the corresponding alkane and replacing it with suffix ene.
Where required, the position of the double bond is specified by appropriate number between the stem name and ending ene except for ethene and propene without positional isomers
Example
Name the following organic compounds using the IUPAC System:

Solution:
(a) Propene (b) But-1-ene (c) But-2-ene (d) 2-methylpropene
(e) 2-methylpent-2-ene (f) 2, 3-dimethylhex-2-ene

Question
(a) Draw the structure of the following organic compounds:
(i) 3, 3-dimethylcyclopentene (ii) 6-bromo-2, 3-dimethylpent-2-ene (iii) 3, 6-dimethyloct-3-ene
(iv) 5-bromo-4-chlorohept-1-ene (v) 4-ethyl-3-methylcyclohexene
(b) Name the following organic compounds:

SYNTHETIC PREPARATIONS OF ALKENES
Alkenes are generally prepared by elimination of one atom or groups from two adjacent carbon atoms mainly from alcohols and halo alkanes.The ease of dehydration of alcohols is: 3o Alcohol> 2o>1oAlcohol while the ease of
dehydrohalogenation of halo alkanes is 3o> 2o>1o
FROM ALCOHOLS
Alcohols are heated with excess concentrated sulphuric acid or orthophosphoric acid. The temperature of dehydration depends on the structure of the alcohol.
The reaction involves removal of water molecules hence known as dehydration reaction
The 1o and 2o Alcohols following the same mechanism while 3o Alcohols undergoes partial ionization generating 3oCarbocations
Example

Example

Example

Write the mechanism of the above reaction.

FROM HALOALKANES

Halo alkanes are heated with a strong base to generate an alkene.
(i) Dehydrohalogenation of alkyl halides.
Halo alkanes are heated with a strong base such sodium amide in liquid ammonia or sodium ethoxide in ethanol in order to remove hydrogen halide i.e.

Example

Example

2. ADDITION OF HALOGENS
   This reaction is usually carried out in an inert solvent such carbon tetrachloride. Chlorine and Bromine reacts but Iodine fails to react.
   The reddish- brown colour of bromine is turned colourless therefore the reaction is used to test for
   presence of carbon-carbon double bond and the number of double bonds since the mole ratio is 1:1(Alkene : Bromine)

Mechanism

N.B. The mechanism is the same when chlorine water is used.

4. ADDITION OF WATER / HYDRATION
   Alkenes react with cold concentrated sulphuric acid to form alkyl hydrogen sulphate which reacts with water on heating alcohol

5. ADDITION OF HYDROGEN HALIDES
   Hydrogen halides react with alkenes forming alkyl halides.

Example

Note. The hydrogen bromide are usually generated by action of concentrated Sulphuric acid on sodium
bromide

6. ADDITION OF HYDROGEN BROMIDE IN PRESENCE OF PEROXIDE
   In this reaction, the bromine atom adds itself to the carbon atom of the double bond having higher number of hydrogen atoms with hydrogen atom (from Hydrogen bromide) to lower carbon atom.
   This observation is known as anti-markovnikov rule.

7. WITH ALKALINE POTASSIUM MANGANATE (VII)
   Manganate (VII) is a weak oxidising agent therefore it oxidizes alkenes to diols. The purple colour turns colourless.

The reaction is also used to test for unsaturated compounds. It can also occur if the reagent is in
acidic medium.

8. OZONOLYSIS
   Alkenes react with ozone to form ozonide which on hydrolysis generates carbonyl compounds. The reaction is used to determine the structural formula of the alkene. Alkenes with terminal double bond form methanol (HCHO), those with structure RCH= form Aldehydes (RCHO) while those of structure RR’C= form Ketones. i.e

Note. Ozonolysis is the formation of ozonide followed by hydrolysis to generated carbonyl compounds.

8. POLYMERISATION REACTION
   Alkenes undergo addition polymerization to form polymers. Ethenepolymerises to form polyethene which is an important plastic material used as packing material.

Sample Question
A hydrocarbon A contains 87.8% carbon. Its relative molecular mass is 82. A decolorizes bromine and in
presence of Nickel it reacts with hydrogen to form B. 0.1g of A was found to absorb 27.3cm3 of hydrogen
(measured at s.t.p). B does not decolorize bromine.
(a) Determine the:
(i) Empirical formula
(ii) Molecular formula of A.
(b) Calculate the:
(i) Number of moles hydrogen that reacted with one mole of A
(ii) Number of double bonds in the molecule of A
(c) Write the possible Isomers of A.

ALKYNES OR ACETYLENES

Alkynes are unsaturated hydrocarbons having a triple bond between carbon atoms. The general formula of the homologous series is CnH2n-2.
NOMENCLATURE OF ALKYNES
The IUPAC names of the alkynes are obtained by taking the corresponding alkane name and replacing
the ending with suffix” –yne”.The position of the triple bond is indicated by inserting the appropriate number between the stem and the ending”–yne”
Question:
Name the following organic compounds using the IUPAC system.

Synthetic Preparation
Alkynes are usually prepared by dehydrohalogenation of dihalogencompounds.
The dihalogen compounds are heated using strong bases which include:
– Sodium ethoxide in ethanol
– Sodium amide in liquid ammonia ( solution of sodium in liquid ammonia)
– Potassium hydroxide in ethanol (contains ethoxide ions but not aqueous Potassium hydroxide or aqueous sodium hydroxide)

Mechanism

Qn: Write the mechanism of the reaction above.
Note: The dihalogen compounds are prepared by:
(a) Bromination or Chlorination of Alkenes

INDUSTRIAL PREPARATION OF ETHYNE
(a) Treatment of Calcium dicarbide with water Calcium dicarbide is obtained by reducing Quicklime, obtained from Limestone, with coke in electric furnace 2000-3000oC

(b) Partial combustion of Methane
In this method, ethyne is separated by passing the gases product through Propanoneor Ammonia

REACTIONS OF ALKYNES
Just like Alkenes, Alkynes also undergo Electrophilic addition due to availability of loosely held electron. Addition of hydrogen, Halogens and Hydrogen halide to alkynes is very much like addition to alkenes except that here two molecules of reagent can be consumed.

Addition of hydrogen

Qn: Write the mechanism of the above reaction

3. Addition of Halogen halides

Qn:Write the mechanism of the above reaction

4. Addition of hydrogen Bromide in presence of peroxides

This reaction resemble that of alkenes to generate anti-markovnikov product

5. Addition of water. Hydration

Ethyne is the only alkyne that forms Aldehydes (ethanal) while the rest form Ketones on hydration.

Polymerization of Ethyne

7. Substitution reaction of Acidic Hydrogen
   Alkynes which have a triple bond at the end of the chain (terminal alkynes) have a replaceable acidic hydrogen atom.
   a) With Ammoniacal Silver nitrate Solution
   Terminal Alkynes react to form white precipitate.

This reagent is used to differentiate terminal alkynes from Non-terminal alkynes
b) With Ammoniacal Copper (I) Chloride solution
Terminal Alkynes react to form red precipitate

Also, ammoniacal copper (I) chloride solution is used to differentiate terminal alkynes from
non-terminal.
c) Sodium Acetylide formation
Terminal Alkynes react with sodium in liquid ammonia to form sodium acetylide which can be used to synthesize the higher homologue by reacting with alkyl halides.

Sample Question
(a) Write equation(s) to show how the following compounds can be synthesized .In each cases indicate the mechanism for the reactions where possible.

(b) Name the reagent(s) that can be used to differentiate between the following compounds. In each case, state what would be observed if the reagent is treated with compounds

When 400cm3 of the mixture of ethane and ethyne measured at room temperature was
bubbled in to mixture of aqueous ammonia and silver nitrate solution. 2.4g of a white solid
was formed.
(i) Write an equation for the reaction that took place and name the white solid
(ii) Calculate the percentage by volume of ethane in the mixture
(d) (i) Define the structural isomerism
(ii) Using suitable example(s), State the classes of structural isomerism