alkanes 1introduction 2nomenclature of alkanes 3physical properties of alkanes 4preparation of...

Alkanes

11 IntroductionIntroduction

22 Nomenclature of AlkanesNomenclature of Alkanes

33 Physical Properties of AlkanesPhysical Properties of Alkanes

44 Preparation of AlkanesPreparation of Alkanes

55 Reactions of AlkanesReactions of Alkanes

Alkanes are the hydrocarbons of aliphatic row.

Alkanes are hydrocarbons in which all the bonds are single covalent bonds (-bonds).

Alkanes are called saturated hydrocarbons.

The simplest one, methane (CH4), is also the most abundant .

All carbon atoms are sp3-hybridized•All bond angles are 109.5°

Alkyl groupsAlkyl groups

Alkyl groups are formed by Alkyl groups are formed by loss of a hydrogen loss of a hydrogen atom from atom from the corresponding alkane . General formula of alkyl the corresponding alkane . General formula of alkyl group : group : CnH2n+1CnH2n+1

( e.g. CH( e.g. CH4 4 Methane – 1 H = -CHMethane – 1 H = -CH3 3 Methyl group )Methyl group )

Alkyl groups are named by dropping the Alkyl groups are named by dropping the -ane -ane suffix of suffix of the alkanes and the alkanes and adding the suffix -yladding the suffix -yl. Methane becomes . Methane becomes a methyl group, ethane an ethyl group, etc. a methyl group, ethane an ethyl group, etc.

Unbranched alkyl groups

Nomenclature Of Branched Alkyl Groups

1. Three-Carbon Groups

- Methylethyl is the systematic name; isopropyl is a common name.

- Numbering always begins at the point where the group is attached to the main chain.

Butane

CH3-CH2-CH2-CH2- CH3-CHH2C

CH3

H3C C

CH3

CH3

n-Butyl Isobutyl tert.Butyl

Propane

CH3-CH2-CH2- CH3-CH

CH3

n-Propyl Isopropyl

2. Four-Carbon Groups

IUPAC Nomenclature

• IUPAC (International Union of Pure and Applied Chemistry) names:

• 1- The unbranched alkanes (homologous series)

• 2- Branched alkanes

• Suffix of Alkanes added ane

1. Straight-Chain Alkanes1. Straight-Chain Alkanes

NameNumber of

carbon atomsStructure

Methane

Ethane

Propane

Butane

Pentane

Hexane

Heptane

Octane

Nonane

Decane

1

2

3

4

5

6

7

8

9

10

CH4

CH3CH3

CH3CH2CH3

CH3(CH2)2CH3

CH3(CH2)3CH3

CH3(CH2)4CH3

CH3(CH2)5CH3

CH3(CH2)6CH3

CH3(CH2)7CH3

CH3(CH2)8CH3

Nomenclature of AlkanesNomenclature of Alkanes

Alkanes

Alkanes are molecules(hydrocarbons) that contain only C-C and C-H bonds.

Alkanes are either acyclic or cyclic.

Acyclic alkanes (not cyclic) have the formula

CnH2n+2 (where n = an integer)

Methane and ethaneare acyclic alkanes.


2. Branched-Chain Alkanes2. Branched-Chain Alkanes

1. Select the longest possible straight chain; this gives the parent name for the alkane

2. Number the parent chain beginning with the end of the chain nearer the branched chain

3. Use the number obtained by application of rule 2 to designate the position of the branched chain

4. When two or more branched chains are present, give each branched chain a number corresponding to its position on the parent chain

5. When two or more branched chains are identical, indicate this by the use of the prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on


H3CHC CH CH2

H2C CH2 CH3

CH3

CH3CH3CH2CH2CH2CHCH3

CH3

CH3CH2CH2CH2CHCH3

CH2

CH3

1. Select the longest possible straight chain; this gives the parent name for the alkane

CH3 CH2 CH2 CH CH CH2 CH3

CH3

CH2 CH37 6 5 4 3 2 1

2. Number the parent chain beginning with the end of the chain nearer the branched chain


2 -methyl hexane 3 -methyl heptane 3,4 -dimethyl heptane

4-ethyl-3- methyl heptane

C

CH3

CH2

CH3

CH CH2 CH2 CH3

CH CH2 CH3H3C

H3C

=>

In writing the full name the In writing the full name the root name root name is placed is placed lastlast; the ; the substituent groupsubstituent group, preceded by the, preceded by the number number indicating its indicating its location location on on the chain, is placed the chain, is placed firstfirst..

14

3- 3- Use the numbers Use the numbers obtained by application of obtained by application of rule 2 rule 2 to to designate the position of the branched chaindesignate the position of the branched chain


4. When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain.

- The substituent groups are listed alphabetically.

5. When two substituents are present on the same carbon, use the number twice.


6. When two or more branched chains are identical, indicate this by the use of the prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on. e.g.


CH3 CH2 C CH2 CH2 CH3

CH3

CH3

1 2 3 4 5 6

3,3- Dimethyl hexane

CH3 CH2 CH CH CH2 CH2 CH2 CH2 CH CH3

CH3CH3

CH3

10 9 8 7 6 5 4 3 2 1

2,7,8-Trimethyl decane



7- When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents.

8. If two substituents are equidistant, look for the next closest group.


1

2

3 4 5

6 7CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

Nomenclature Of Cycloalkanes

Monocyclic Compounds

1. Cycloalkanes with only one ring:

1) Number the ring beginning with the substituent first in the alphabet, and number in the direction that gives the next substituent the lower number possible.

2) When three or more substituents are present, begin at the substituent that leads to the lowest set of locants.

Substituted cycloalkanes:

• 1. A series of compounds, where each member differs from the next member by a constant unit, is called a homologous series. Members of a homologous series are called homologs.

• 2. At room temperature ( 25 °C) and 1 atm pressure, the C1-C4 unbranched alkanes are gases; the C5-C17 unbranched alkanes are liquids; the unbranched alkanes with 18 or more carbon atoms are solids.

Physical Properties Of Alkanes and Cycloalkanes

Physical Properties of Alkanes

Boiling PointBoiling Point

• Higher members have higher boiling points

Reason:

• Increase in molecular mass

• Increase in intermolecular force


Branched-chain alkanes have lower boiling points than straight-chain alkanes

∵molecule is more compact surface area van der Waals’ force boiling point


Melting PointMelting Point •Higher members have higher (less for odd- number of carbons)melting points

Reason:

• Increase in molecular mass

• Increase in intermolecular force

Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.


SolubilitySolubility

Alkanes:

• non-polar compounds

• insoluble in water and highly polar solvents

• soluble in non-polar solvents like benzene, 1,1,1-trichloroethane

Alkanes are less dense than water and swim on top of water

DensityDensity

All alkanes and cycloalkanes have densities less than 1 g cm–3 at 20°C.

Petroleum floats on water surface

Cycloalkanes• Rings of carbon atoms (CH2 groups)

• Formula: CnH2n

• Nonpolar, insoluble in water

• Compact shape

• Melting and boiling points similar to branched alkanes with same number of carbons

• 5- and 6-membered rings most stable

• Bond angle closest to 109.5

CH2 CH2

Ni or Pd or Pt / H2

200, 300CH3 CH3

Hydrogenation of unsaturated hydrocarbon:Hydrogenation of unsaturated hydrocarbon:

2) From alkyl Halides

B) Hydrolysis of Grignard Reagent

CH3CH2Br + Mg2+ Dry ether CH3CH2MgBr

Grignard reagent

CH3CH2MgBrH3O

+

CH3CH3 + Mg(OH)Br

Grignard reagent is an alkyl magnesium halide compound, R-Mg-X

- The Grignard reagent is formed when a solution of an Alkyl Halide (R-X) is allowed to stand over a metallic magnesium in the presence of dry ether

- Then Grignard reagent react with water or alcohol to form alkane

C) Wurtz Reaction تفاعل فورتز

2 CH3 CH2 CH2 Br + 2 Na CH3 CH2 CH2 CH2 CH2 CH3

+ 2 NaBr

This is the reaction of two alkyl halides (R-X) with metallic sodium to give symmetrical alkanes.

The wurtz reaction is a poor method for the preparation of unsymmetrical alkanes

Preparation of Alkanes ReformingReforming

Reforming is a process in which straight-chain alkanes are heated under pressure in the presence of a platinum catalyst. The chains break up and reform to give branched-chain molecules.

29.5 Reactions of Alkanes

Complete combustion :

Alkanes react with sufficient oxygen to give carbon dioxide and water through a complex series of reaction with the release of a large amount of energy.

General formula:

CombustionCombustion

OH1CO O2

13HC 22222

nn

nnn

complete combustion

CH3-CH2-CH3 + 4 O2 ——> CO2 + 2 CO + 4 H2O + heat

incomplete combustion

Reactions of Alkanes

• Methane reacts with chlorine under diffuse sunlight or heating but not in dark

• A mixture of products (CH3Cl, CH2Cl2, CHCl3, CCl4) is

formed with the replacement of hydrogen by one or more chlorine atom

ChlorinationChlorination

)HCl()Cl(CH )(Cl)(CH 3light UV

24 gggg

)HCl()(ClCH )(Cl)(ClCH 22light UV

23 gggg

)HCl()(CHCl )(Cl)(ClCH 3light UV

222 gggg

)HCl()(CCl )(Cl)(CHCl 4light UV

23 gggg

• If methane is in excess, CH3Cl is

major product

• If chlorine is in excess, CCl4

is major product


Reaction Mechanism: Free Radical Substitution Reaction

Mechanism of reaction :

1. Chain initiation

• homolytic fission of chlorine molecules by heat or light into two chlorine radicalsStep 1:


2. Chain propagation

Step 2:

Step 3:

• steps 2 and 3 repeat hundreds or thousands of time due to formation of the reactive intermediate in each step chain reaction


Further substitution occurs:


3. Chain termination

• two free radicals combine to form a neutral molecule

• the chain reaction is terminated

ExampleExample

Write down the reaction mechanism involved in the chlorination of ethane in the presence of diffuse sunlight.

Answer


Solution:

The reaction mechanism is shown as follows:

1. Chain initiation

2.Chain propagation


Solution:

3. Chain termination

Mechanism For the Radical Halogenation of Methane

alkanes 1introduction 2nomenclature of alkanes 3physical properties of alkanes 4preparation of...

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