Alkenes, Alkynes and aromatic compounds

Hydrocarbons (contain only carbon and hydrogen)

a) Saturated: (Contain only single bonds)Alkanes (CnH2N + 2 )Cycloalkanes (CnH2N )

b) Unsaturated: contain Alkenes: double bonds (,,,CnH2N) Alkynes: triple bonds ((CnH2N - 2)Aromatic: benzene like compounds1

Facts about double and triple bonds

HH

bond angle 109.5 120 o 180 o

bond length 154 pm 134 pm 121 pm

rotation possible restricted restricted

geometry tetrahedral triagonal planer linear

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Biologically active alkene derivatives

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α-Linolenic acid

Linoleic acid

Arachidonic acid

Isoprene = 2-Methyl-1,3 -butadiene

Nomenclature of alkenes and Alkynes1) The ending ene is used for alkenes and yne for alkynes2) Select the longest chain that includes both carbons of the

multiple bond3) Number the chain from the end nearest to the multiple bond4) Indicate the position of the multiple.

Examples

CH3

CH3

CH3

BrCH3

CH21

12 3

45

4-bromo-2-methyl-2-pentene

1

23456

2-propyl-1-hexene6

Isomers and common names of simple alkenes

H

HH

H Cl

HH

H

H

HH

CH3 H

HH

CH2

Cl

CH2

HCH3

H CH3

HH

CH3 H

CH3H

CH3

CH3

CH2

CH3

Ethene (ethylene) vinyl chloride

Propene (propylene) Allyl chloride

cis-2-butene trans-2-butene 2-methyl-1-propene (isobutene)1-butene 7

Nomenclature of compounds containing more than one multiple bond1) Number from the end nearest to the multiple bond.If a double and a triple bond are equidistant from he end, the

double bond receives the lowest numbersExample

123456

7 123456

7

2-hepen-4-yne 4-hepten-2-yne

123456

7

2-hepten-5-yne not 5-hepten-2-yne8

Compounds containing more than one double bond

CH3CH3 Br

1 2 3 41

23

4

5 12

34

5

6

1234

567

12

3

4

5 6

7

8

2-methyl-1,3-butadiene 3-methylcyclopentene 5-bromo-1,3-cyclohexadiene

1,3,5-heptatriene 1,3,5-cyclooctatriene

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Classification according to the position of double bonds

Cis-Trans (Z-E) Isomerism in Alkenes

Because rotation at carbon–carbon double bonds is restricted, cis–trans isomerism (geometric isomerism) is possible.If the two groups are identical we distinguish the two isomers by adding the prefix cis (same side) or trans (opposite sides) Example

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If the groups attached to the double bond are different, we distinguish the two isomers by adding the prefix Z (same side) or E (opposite sides) depending on the atomic number of the atoms attached to each end of the double bond

CH3

BrI

ClBr

CH3I

Cl

CH

CH3

O

OH

CH3

I

Cl

H

CH2

Z-2-bromo-1-chloro-1-iodopropene E-2-bromo-1-chloro-1-iodopropene

Z Z

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Reactions of Alkenes

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The most common reaction of alkenes is addition of a reagent to the carbons of the double bond to give a product with a C-C single bond.

What is the difference between addition and substitution reactions?

Substitution :

Addition:

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Example for reaction a:

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Example 1 for reaction b :

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When an unsymmetric reagent adds to an unsymmetric alkene, the electropositive part of the reagent bonds to the carbon of the double bond that has the greater number of hydrogen atoms attached to it

Example 2 for reaction b:

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Example for reaction c:

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c.

Example for reaction C:

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A polymer is a large molecule, usually with a high molecular weight, built up from small repeating units. The simple molecule from which these repeating units are derived is called a monomer, and the process of converting a monomer to a polymer is called polymerization.

d.

Reactions of Alkynes

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22

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Examples to reactions a:

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Aromatic Compounds

Benzene, C6H6, is the parent hydrocarbon of the especially stable compounds known as aromatic compounds .

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Nomenclature of Aromatic Compounds

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Pres

erva

tive

for u

rine

spec

imen

s

Nomenclature of Aromatic CompoundsWhen two

substituents are present, three isomeric structures are possible. They are designated by the prefixes ortho- , meta-, and para, which are usually abbreviated as o- , m-, and p- , respectively.

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Examples :

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Examples :

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The most common reactions of aromatic compounds involve substitution of other atoms or groups for a ring hydrogen on the aromatic unit. Here are some typical substitution reactions of benzene.

Reactions of aromatic compounds

Polycyclic Aromatic Hydrocarbons

Naphthalene is a planar molecule with two fused benzene rings. The two rings share two carbon atoms.

Phenanthrene is an isomer of anthracene and is considered to be the basic structure of many important compounds like cholesterol, male and female sex hormones, and Vitamin D.

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Some polycyclic aromatic compounds such as benzo[a]pyrene are cancerogenic

Cancerogenic compounds can produce a tumor on mice in a short time when only trace amounts are painted on the skin.

These carcinogenic hydrocarbons are present not only in coal tar but also in tobacco smoke and can be formed in barbecuing meat

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