alkane onwards

7/30/2019 Alkane Onwards

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Hydrocarbons

Lecture no. 29-32


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Organic Compounds

Contain carbon

Have covalent bonds

Have low melting points

Have low boiling points

Burn in air (oxygen)

Generally soluble in nonpolar solvents

Form large molecules


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24.1

Common Elements in Organic Compounds


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Classification of organic compounds:

Classes of organic compounds could be distiguished according to

the functional group that the compound contains.

A functional group is a group of atoms that is largely responsible

for the chemical behavior of the parent molecule.

Most organic compounds are derived from hydrocarbons.

On the basis of structure hydrocarbons are divided into two main

classes- aliphatic and aromatic.

Aliphatic hydrocarbons are further divided into: alkanes, alkenes,

alkynes and cycloalkanes.


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24.1

Classification of Hydrocarbons


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Alkanes

Contain C and H only

Contain single bonds C-C

Have 4 bonds to every carbon (C) atom

Are nonpolar


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Alkanes

Alkanes have the general formula CnH2n+2 where n= 1,2,3,

only single covalent bonds

saturated hydrocarbons because they contain the maximum

number of hydrogen atoms that can bond with the number of carbon

atoms in the molecule

CH4 C2H6 C3H8

methane ethane propane


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10/6124.2

Structural isomers are molecules that have the same molecular

formula but different structures


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How many structural isomers does pentane, C5H12, have?

C C C C C

H H H H H

H

HHHHH

H

C C C C

H CH3 H H

H

HHHH

H

C C C

H CH3 H

H

HH

H

CH3

n-pentane

2-methylbutane

2,2-dimethylpropane

24.2


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Isomers could only be distinguished by their systematic names or

IUPAC nomenclature.Decan has 75 isomers.


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Alkane Nomenclature

1. The parent name of the hydrocarbon is that given to the longest

continuous chain of carbon atoms in the molecule.

CH3 CH2 CH2 CH CH2 CH2 CH3

CH3

1 2 3 4 5 6 7

4-methylheptane

2. An alkane less one hydrogen atom

is analkylgroup.

CH4

CH3

methane

methyl


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24.2

Alkane Nomenclature

3. When one or more hydrogen atoms are replaced by other groups,

the name of the compound must indicate the locations of carbonatoms where replacements are made. Number in the direction that

gives the smaller numbers for the locations of the branches.

CH3 CH CH2 CH2 CH3

CH3

1 2 3 4 5

2-methylpentane

CH3 CH2 CH2 CH CH3

CH3

1 2 3 4 5

4-methylpentane


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24.2

Alkane Nomenclature

4. Use prefixes di-, tri-, tetra-, when there is more than one alkyl

branch of the same kind.

CH3 CH CH CH2 CH2 CH3

CH3

1 2 3 4 5 6

CH3

2,3-dimethylhexane

CH3 CH C CH2 CH2 CH3

CH3

1 2 3 4 5 6

CH3

3,3-dimethylhexane


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24.2

Alkane Nomenclature

5. Use previous rules for other types of substituents.

CH3 CH CH CH3

Br

1 2 3 4

NO2

2-bromo-3-nitrobutane

CH2 CH2 CH CH3

Br

1 2 3 4

NO2

1-bromo-3-nitrobutane


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What is the IUPAC name of the following compound?

24.2

1 2 3 4 5 6 7 8

CH3 CH CH2 CH CH2 CH2 CH3

C2H5

CH2

CH3

4-ethyl-2-methyloctane

What is the structure of 2-ethyl-4-methylhexane?

1 2 3 4 5 6CH3 CH CH2 CH CH2 CH3

CH3C2H5


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As low number as

possible


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Sum of the numbers

should be minimum


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24.2

achiral chiral

Optical isomerism in alkanes


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24.2

Cycloalkanes

Alkanes whose carbon atoms are joined in rings are called

cycloalkanes. They have the general formula CnH2n where n= 3,4,


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Cycloalkanes

24.2
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24.2

Alkenes

Alkenes have the general formula CnH2n where n= 2,3,

contain at least one carbon-carbon double bond

also calledolefins

CH2 CH CH2 CH3

1-butene

CH3 CH CH CH3

2-butene

C C

Cl Cl

H H

C C

Cl H

H Cl

cis-dichloroethylene trans-dichloroethylene


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prepared by Dr. Harpreet Kaur


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Nomenclature of alkene:


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Cis-trans isomerism


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Cis-Trans Isomerization in the Vision Process

24.2

Alk


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24.2

Alkynes

Alkynes have the general formula CnH2n-2 where n= 2,3,4,

contain at least one carbon-carbon triple bond

1-butyne 2-butyne

CH C CH2 CH3 CH3 C C CH3

Production of acetylene

CaC2(s) + 2H2O (l) C2H2(g) + Ca(OH)2(aq)


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sp


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Reactions of alkanes: glance over types of reactions

Alkanes are generally considered unreactive, but under favorable

conditions they do react

Alk R ti
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24.2

Alkane Reactions

CH4(g) + 2O2(g) CO2(g) + 2H2O (l) DH0 = -890.4 kJ

Combustion

Halogenation

CH4(g) + Cl2(g) CH3Cl (g) + HCl (g)light

Cl2 + energy Cl

+ Cl

Cl + C HH

H

H

C H

H

H

+ HCl

CH

H

H

+ Cl Cl C ClH

H

H

+ Cl


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Reactions of alkenes

Weak electrons are attracted by electrophile and than end of

reaction is carried out by nucleophile

C2H6(g) CH2 CH2(g) + H2(g)Pt

catalyst

Cracking(prepration of alkenes)


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Markovnikovs rule: the hydrogen adds to sp2 carbon that is

bonded to greater no ofhydrogens


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Alkene Reactions


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24.2

Alkene Reactions

Addition Reactions

CH2 CH2(g) + HBr (g) CH3 CH2Br (g)

CH2 CH2(g) + Br2(g) CH2Br CH2Br (g)

Alkyne Reactions


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24.2

Alkyne Reactions

Addition Reactions

Hydrogenation

CH CH (g) + H2(g) CH2 CH2(g)

CH CH (g) + HBr (g) CH2 CHBr (g)

CH CH (g) + Br2(g) CHBr CHBr (g)

CH CH (g) + 2Br2(g) CHBr2 CHBr2(g)


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H3C C C CH3

xs H2

Pt

CH3CH2CH2CH3

H 2/Pd/BaSO 4

Quinoline

(Lindlar's Catalys t)

H3C

C

H

C

H

CH3

Na NH 3 (liq)

H3C

C

H

C

H

CH3

Hydrogen Halide


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Hydrogen Halide

Addition to AlkynesHeteroatom stabilized carbocation

intermediates

H3C C C HHX

H3C C C H

H

+

X-

H3C C C H

H

X

Markovnikov

addition

a vinyl halide

H3C C C H

H

X

HXH3C C C H

H

HX

+ X-

a heteroatom

stabilized carbocation

H3C C C H

H

HX+

H3C C C H

H

X H

X

a geminal dihalide


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Halogenation of Alkynes

Formation of Vicinal tetrahalides

C C

an alkyne

X2 C C

X X

X X

a vicinal tetrahalide

carbonte trachloride

S b tit ti


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Substitution

Alkynyl Anion Synthesis of Alkynes via

Bimolecular Nucleophilic Substitution

R C CHNa

R C C Na+

R C C Na+ + R'CH2 X R C C CH2R' + NaX

+ H2

TerminalAlkyne

Methylor

Primary

Alkyl

Halide


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Diels-Alder Reaction

Six-membered Ring formation (4+2) Diene + Dienophile

CC

CC

C

C

Examples of reactions in Alkynes


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Examples of reactions in Alkynes


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Chemistry In Action: Ice That Burns


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y

alkane onwards

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