Hydrocarbons

Those compounds which contain only carbon

and hydrogen.

There are two types of hydrocarbons:

1- saturated hydrocarbons: in which all

carbon bonds are sigma (σ)bonds,

include alkanes and cycloalkanes.

2- unsaturated hydrocarbons: which contain sigma

as well as pi-bonds, include alkenes (paraffin) and

Alkynes (acetylenes).

Nomenclature of organic compounds:

The IUPAC rules for alkanes

1.The general name for acyclic saturated hydrocarbon is Alkanes. The – ane ending is used for all saturated hydrocarbons

2: alkanes without branches are named according to the number of carbon atoms.

CH4 MethaneCH3CH3 EthaneCH3CH2CH3 PropaneCH3(CH2)2CH3 n- butaneCH3(CH2)3CH3 n- pentaneCH3(CH2)4CH3 n- hexaneCH3(CH2)5CH3 n- heptaneCH3(CH2)6CH3 n- octane CH3(CH2)7CH3 n- None

3: for alkanes with branched chains, the root name

Is that of the longest continuous chain of carbon atoms, for example

CH3CH CHCH2CH3

CH3CH3

The longest continuous chain (colored C’s) has five carbon atoms .the compound is therefore named as substituted pentane.

4: Groups attached to the main chain are called substituents. Saturated substituens that contain only carbon and hydrogen called alkyl groups

Alkyl groups

- CH3 Methyl

- CH2CH3 Ethyl

- CH2CH2CH3 n-Propyl

CH3CHCH3 isopropyl

Other substituent• -OH• -NO2

• -OCH3

• -CN• -Cl• -Br• -F• -I• -NH2

• Hydroxy• Nitro• Methoxy• Cyano• Chloro• Bromo• Flouro• Iodo• Amino

In the example, each branch has only one carbon. derived from the hydrocarbon methane by removing one of the hydrogens, it is called the methyl group.

CH3- or Me-

5: groups are located by a name and number.

The main chain is numbered in such a way that the first substituent receive the lowest possible number.

`

CH3 CH CH CH2 CH3

CH3 CH3

1 2 3 4 5

When two identical groups are attach to the main chain, prefixes such as di, tri, and tetra- are used.

The previous compound is correctly named as 2,3 - dimethylpentane

6: Punctuation is important in writing IUPAC names .The names are written as one word. Numbers are separated from each other by commas and from letters by hyphens.

If two or more different types of substituents are present, they are listed alphabetically, except that prefixes such as di- and tri- are not counted for alphabetizing.

Examples:

Give the IUPAC name for the following compounds.

A: CH3 CH CH2 CH3

CH3 CH3

B: CH3 CH2 CH CH3 C: CH3– C-CH3

NO2 CH3

D) CH3C(CH3)3 E) BrCH2CH2CHCl2

Cycloalkanes

Carbocyclic hydrocarbons that are named by placing the prefix cyclo- before the alkane name corresponding to the number of carbon atoms in the ring.

Cyclopropane cyclobutane cyclopentane

Cyclohexane cyclooctane

When the ring carry alkyl or halogen substituents, they are named in the usual way.

CH3 CH3 CH3 CH3

CH3

Methylcyclopropane 1,1-dimethylcyclopentane 1,2-dimethylcyclopentane

Alkenes

are hydrocarbons that contain one

double bond, When the molecule contain

two double bonds it is often called a diene

CH2=CH2 Ethene (ethylene)

CH2=CH-CH3 Propene

CH2=CH-CH2-CH3 Butene-1

CH3-CH=CH-CH3 Butene-2

cyclopentene

CH3

3-methylcyclohexene

CH3

1-methylcyclopentene

Preparation of alkenes

The most used methods for preparations of alkenes and alkynes are based on elimination reactions

C C

Y Z

eliminationC C

alkene

1- dehydrogenation This reaction includes removal of hydrogen from saturated hydrocarbons in presence of a catalyst and at high temperature e.g

CH3

CH3Cr2O3/Al2O3

CH3

CH3+ CH2

CH3

+ H2

2-dehalogenation

The reaction includes removal of halogen of 1,2- dihalogen derivatives of alkanes by means of Zn dust and methanol

CHBr

CH3

BrH2C

ZnCH3OH CH3-CH=CH2 + ZnBr2

3- Dehydration:

Dehydration of secondary and tertiary alcohol using dilute sulphuric acid

CH3CH2CHOHCH3 CH3CH=CHCH3 + H2O

4- Dehydrohalogenation

By the action of ethanolic potassium hydroxide on alkyl halides, eg propene from propyl bromide

CH3CHBrCH3 CH2=CHCH3

CH3CH2CHBrCH3 CH3CH=CHCH3

KOH

KOH

Chemical properties of alkenes

Owing to the presence of the double bond

the alkenes undergo addition reactions

C = C + E – Nu C+ CE

CNu CE

1-Hydrogenation

Alkenes are hydrogenated under

pressure of hydrogen at the presence

of a catalyst

CH2 = CH2 + H2 CH3 - CH3

catalyst

2- Hydration

Alkenes are catalytically hydrated in dilute acid solution giving alcohols

CH3CH =CH2 + H2O CH3CHOHCH3H+

3- Halogenation Chlorine and bromine are readily adde to the double bond in the dark

CH2 = CH2 + Br2 CH2Br – CH2Br

alkynesNomenclatures

Alkynes

Name Formula Structural Formula

Ethyne (Acetylene)

C2H2 CH ≡CH

Propyne C3H4 CH≡ C-CH3

1-Butyne C4H6 CH≡ C-CH2-CH3

2-Butyne C4H6 CH3-C≡ C-CH3

General methods for preparation of alkynes

1- By the action of potassium hydroxide on gem or Vicinal dihalide the reaction proceeds in two steps

BrCH2CH2Br + KOH CH2=CHBr

CH2= CHBr + KOH CH≡CH+2KBr+H2O Acetylene

ethanol

ethanol

2- Dehalogenation

By the action of zinc dust and heat on tetrahalogeno alkanes

CH3CBr2-CHBr2 + 2 Zn CH3 – C≡ CHHeat -2ZnBr2

Chemical properties of alkynes :

Undergo the same reaction as alkenes but using two moles of the reagent

1- CH3C = CH CH3CH2CH3

Propyne n- propane

2- CH = CH + Cl2CH-CHCl2

3- CH3C = CH CH3CBr2CH3

2H2/Pd

2HBr

2Cl2

Aromatic hydrocarbons

Aromatic are cyclic, they are unsaturated,

usually have three conjugated double bonds for

each ring of carbon atoms , not all aromatic are

hydrocarbon e.g pyridine

N

Pyridine

Benzene

Benzene is the an example for aromatic

hydrocarbons.

Benzene has six pi electrons two from each

of the double bonds

benzene

Fused ring system

Those are aromatic similar to benzene with more

than one benzene ring e.g naphthalein and

anthracene

naphthaleneanthracene

Substituted benzene

The benzene ring minus one hydrogen (C6H5-)

is named as phenyl group. In general group

derived from aromatic compounds by removing

One hydrogen from carbon called aryl group.

When one hydrogen replaced by other group is

called monosubstituted

Cl CH3O OH O H

NH2

O NH2OCH3

chlorobenzene benzoic acid benzaldehyde

aniline benzamide anisole

toluene

ClCl

CH3CH3

O OH

OH

o-xylene1,2-dichlorobenzene

2-hydroxybenzoic acid

Reactions of Benzene •Benzene is always stable because of its delocalized (π) bonds. •Benzene is therefore chemically less reactive than unsaturated aliphatic hydrocarbons.

+ Br2F e B r2

Br

+ HBr

(1) Halogenation:

+ HNO3

NO2

+ H2OH 2S O 4

(2) Nitration:

(3) Sulphonation:

+

SO3H

+ H2OH 2S O 4 Benzene sulphonic acid

+

CH3

+ ClHCH3Cl

(4) Alkylation: