Alkenes and Electrophilic Addition

A-Level Book 3A A-Level 3A1

Alkenes and Alkenes and Electrophilic Electrophilic

AdditionAddition2288

28.128.1 IntroductionIntroduction

28.228.2 Nomenclature of AlkenesNomenclature of Alkenes

28.328.3 Physical Properties of AlkenesPhysical Properties of Alkenes

28.428.4 Preparation of AlkenesPreparation of Alkenes

28.528.5 Reactions of AlkenesReactions of Alkenes

28.28.11IntroductionIntroduction

28.1 Introduction (SB p.167)

AlkeneAlkeness• Unsaturated hydrocarbons containing the

C=C double bond

• General formula of alkenes: CnH2n

• The carbon atoms involved in the C=C double bond are sp2-hybridized

28.1 Introduction (SB p.167)AlkeneAlkeness• The C=C double bond is made up of a

bond and a bond

• Trigonal planar geometry

• Bond angle = 120o

The C=C double bond in ethene

AlkeneAlkeness bond can be broken down easily

alkenes are reactive compounds

undergo mainly addition reactions

AlkeneAlkeness• Limited rotation of the C=C double bond

alkenes show geometrical isomerism

• e.g.

28.28.22NomenclaturNomenclature of Alkenese of Alkenes

28.2 Nomenclature of Alkenes (SB p.167)

IUPAC Rules of Naming of IUPAC Rules of Naming of AlkenesAlkenes

1. Select the longest possible straight chain that contains the C=C double bond

determine the stem name

use the ending of “-ene”

2. Number the parent chain so as to include both carbon atoms of the double bond

Begin numbering with the end of the chain nearer the C=C double bond

3. Designate the position of the C=C double bond by using the number of the first atom of the double bond

4. Designate the position of the substituents by using the numbers obtained

IUPAC Rules of Naming of IUPAC Rules of Naming of AlkenesAlkenese.g.

5. If two identical groups are present on the same side of the C=C double bond

the compound is designated as cis

If they are on opposite sides

the compound is designated as trans

IUPAC Rules of Naming of IUPAC Rules of Naming of AlkenesAlkenese.g.

Check Point 28-2Check Point 28-2Example 28-2Example 28-2

28.28.33 Physical Physical Properties of Properties of

AlkenesAlkenes

28.3 Physical Properties of Alkenes (SB p.169)Physical Properties of Physical Properties of AlkenesAlkenes

Some physical properties of several alkenes

Name Condensed structural formula

Boiling point (oC)

Melting point (oC)

Density at 20oC

(g cm-3)

Ethene CH2=CH2 -104 -169

Propene CH3CH=CH2 -47.7 -185 0.514

But-1-ene CH3CH2CH=CH2

-6.3 -185 0.595

Pent-1-ene

CH3CH2CH2

CH=CH2

30 -165 0.641

Hex-1-ene

CH3CH2CH2

CH2CH=CH2

62.9 -140 0.673

28.3 Physical Properties of Alkenes (SB p.169)Physical Properties of Physical Properties of AlkenesAlkenes

Some physical properties of several alkenes

Name Condensed structural formula

Boiling point (oC)

Melting point (oC)

Density at 20oC

(g cm-3)

cis-But-2-ene

CH3CH=CHCH3 (cis)

4 -139 0.621

trans-But-2-ene

CH3CH=CHCH3 (trans)

1 -106 0.604

2-Methylbut-1-ene

CH3CH2C(CH3)=CH2

31 -138 0.650

28.3 Physical Properties of Alkenes (SB p.169)

Physical Properties of Physical Properties of AlkenesAlkenes• Alkenes are non-polar

• Dissolve in non-polar solvents or in solvents of low polarity

• Only very slightly soluble in water

28.3 Physical Properties of Alkenes (SB p.169)

Physical Properties of Physical Properties of AlkenesAlkenes

• M.p. and b.p of alkenes are lower than their corresponding alkanes

• Densities of alkenes are less than that of water

28.28.44Preparation Preparation

of Alkenesof Alkenes

28.4 Preparation of Alkenes (SB p.170)

CrackinCrackingg• Prepared by the cracking of alkanes of

high molecular masses

• Give alkenes of low molecular masses

CrackinCrackingg

2CH3CH3 CH2 = CH2 + 2CH4600 oC

2CH3CH2CH3

CH3CH = CH2 + CH2 = CH2 + CH4 + H2

600 oC

Elimination ReactionsElimination Reactions

• Involve removal of atoms or groups of atoms from adjacent carbon atoms in the reactant molecule

• Formation of a double bond between carbon atoms

1. Dehyhalogenation

1. Dehyhalogenation• Elimination of a hydrogen halide

molecule from a haloalkane

• By heating the haloalkane in an alcoholic solution of KOH

1. Dehyhalogenation

1. Dehyhalogenatione.g.

1. Dehyhalogenation

1. Dehyhalogenation• The ease of dehydrohalogenation of

haloalkanes decreases in the order:

Tertiary haloalkane

Secondary haloalkane

Primaryhaloalkane

1. Dehyhalogenation

1. Dehyhalogenation• Different classes of haloalkanes or

alcohols have different reactivities

• May undergo different types of reactions under the same reaction conditions

1. Dehyhalogenation

1. Dehyhalogenation• Dehydrohalogenation of secondary or

tertiary haloalkanes can take place in more than one way

• A mixture of alkenes is formed

1. Dehyhalogenation

CH3CH2CHClCH3

CH3CH=CHCH3 + CH3CH2CH=CH2

2-Chlorobutane

But-2-ene (80 %) But-1-ene (20 %)

alc. KOH

• The more highly substituted alkene (i.e. but-2-ene) is the major product

1. Dehyhalogenation

1. Dehyhalogenation• The more highly substituted alkene is

the alkene with a larger number of alkyl groups bonded to the C = C group

• The greater the number of alkyl groups that an alkene contains

the more stable the molecule

1. Dehyhalogenation

1. Dehyhalogenation• The relative stabilities of alkenes

decrease in the order:

2. Dehydration of Alcohols2. Dehydration of Alcohols• Removal of a water molecule from a

reactant molecule

• By heating the alcohols in the presence of concentrated sulphuric acid

• Give alkenes and water as the products

2. Dehydration of Alcohols2. Dehydration of Alcohols• Experimental conditions (i.e. temperature

and concentration of concentrated sulphuric acid)

closely related to the structure of the individual alcohol

2. Dehydration of Alcohols2. Dehydration of Alcohols

• Primary alcohols generally required concentrated sulphuric acid and a relatively high temperature

2. Dehydration of Alcohols2. Dehydration of Alcohols• Secondary alcohols are intermediate in

reactivity

• Tertiary alcohols dehydrate under mild conditions (moderate temperature and dilute sulphuric acid)

• The relative ease of dehydration of alcohols generally decreases in the order:

Tertiary alcohol

Secondary alcohol

Primaryalcohol

• Secondary and tertiary alcohols dehydrate to give a mixture of alkenes

• The more highly substituted alkene is formed as the major product

Example 28-4Example 28-4 Check Point 28-4Check Point 28-4

Addition Addition ReactionsReactionsHydrogenationHydrogenation• Alkenes can be prepared by hydrogenation

of alkynes

Depend on the conditions and the catalyst employed

HydrogenationHydrogenation

• Lindlar’s catalyst is metallic palladium deposited on calcium carbonate

further hydrogenation of the alkenes formed can be prevented

28.528.5Reactions of Reactions of

AlkenesAlkenes

28.5 Reactions of Alkenes (SB p.174)

Why do Alkenes Undergo Addition Why do Alkenes Undergo Addition Reactions Readily?Reactions Readily?

• Presence of C=C double bond

• C=C double bond is made up of a bond and a bond

• In addition reactions,

one bond and one bond are broken

two bonds are formed

• Heat evolved during bond formation >Heat required during bond breaking

• Addition reactions are usually exothermic

• The electrons of the bond are

diffuse in shape

less firmly held by the bonding carbon

nuclei

• Susceptible to the attack by electrophiles

• Electrophiles that attack the C=C double bond

protons (H+)

neutral species in which the molecule is polarized, e.g. bromine

Electrophilic Addition Electrophilic Addition ReactionsReactions• Addition of electrophiles to the C=C

double bond of alkenes

1. Addition of Hydrogen Bromide1. Addition of Hydrogen Bromide• A molecule of HBr adds to the C=C

double bond of an alkene

• Give a bromoalkane

1. Addition of Hydrogen Bromide1. Addition of Hydrogen Bromidee.g. the addition of HBr to

ethene produces bromoethane

1. Addition of Hydrogen Bromide1. Addition of Hydrogen Bromide• When but-2-ene reacts with HBr

2-bromobutane is formed as the only product

1. Addition of Hydrogen Bromide1. Addition of Hydrogen Bromide• When propene reacts with HBr

the major product is 2-bromopropane

the minor product is 1-bromopropane

Reaction Mechanism: Electrophilic Reaction Mechanism: Electrophilic Addition Reactions of Hydrogen Addition Reactions of Hydrogen Bromide to AlkenesBromide to Alkenes• Step 1:

The alkene abstracts a proton from hydrogen bromide

form a carbocation and a bromide ion

Reaction Mechanism: Electrophilic Reaction Mechanism: Electrophilic Addition Reactions of Hydrogen Addition Reactions of Hydrogen Bromide to AlkenesBromide to Alkenes• Step 2:

The bromide ion reacts with the carbocation by donating an electron pair

a bromoalkane is formed

Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule

CH3CH=CHCH3 is a symmetrical alkene.

CH3CH=CH2 is an asymmetrical alkene.

• A hydrogen halide can add to an asymmetrical alkene in either of the two ways

• The reaction proceeds to give a major product preferentially

the reaction is said to exhibit “regioselectivity”

Markovnikov’s rule states that in the addition of HX to an asymmetrical alkene, the hydrogen atom adds to the carbon atom of the carbon-carbon double bond that already has the greater number of hydrogen atoms

• The products formed according to this rule are known as Markovnikov products

Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• Carbocations are a chemical species that

contains a positively charged carbon

• Very unstable

• Exist transiently during the reaction

• Classified as primary, secondary or tertiary

according to the number of carbon chains that are directly attached to

the positively charged carbon

Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• Carbocations are a reactive intermediate

formed during the reaction

react to give the product, or

convert back to the reactant

• The more stable the carbocation

the faster its formation

Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• The stability of the carbocations

increases in the order:

Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• Alkyl groups stabilize the positively

charged carbocation by positive inductive effect

• A greater number of alkyl groups

release more electrons to the positively charged carbon

increase the stability of the carbocation

Check Point 28-5ACheck Point 28-5A

Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• Consider the addition of HBr to propene:

Stability of Carbocation and Stability of Carbocation and Mechanistic Explanation of the Mechanistic Explanation of the Markovnikov’s RuleMarkovnikov’s Rule• The hydrobromination of propene

involves two competing reactions:

2. Addition of Halogens2. Addition of Halogens• Halogens normally react with alkenes by

electrophilic addition

where X2 can be F2, Cl2 or Br2

2. Addition of Halogens2. Addition of Halogens• Alkenes react rapidly with Cl2 (or Br2) in

1,1,1-trichloroethane at room temp and in the absence of light

• Form dichloroalkanes (or dibromoalkanes)

2. Addition of Halogens2. Addition of Halogens

2. Addition of Halogens2. Addition of Halogense.g.

2. Addition of Halogens2. Addition of Halogens• The decolourization of bromine in 1,1,1-

trichloroethane is a useful test for unsaturation

A drop of bromine

dissolved in 1,1,1-

trichloroethane is added to

an alkene

The reddish brown

colour of bromine is decolourize

3. Addition of Bromine Water (HOBr)3. Addition of Bromine Water (HOBr)• In an aqueous solution of bromine, the

following equilibrium is established:

Br2 + H2O HBr + HOBr

Bromic(I) acid

3. Addition of Bromine Water (HOBr)3. Addition of Bromine Water (HOBr)• Bromic(I) acid reacts readily with an

alkene at room conditions to form a bromohydrin

3. Addition of Bromine Water (HOBr)3. Addition of Bromine Water (HOBr)e.g.

• The consequent decolourization of the reddish brown colour of bromine water is also a test for unsaturation

4. Acid-catalyzed Hydration4. Acid-catalyzed Hydration

• Alkenes dissolve in cold and concentrated sulphuric acid

4. Acid-catalyzed Hydration4. Acid-catalyzed Hydratione.g.

4. Acid-catalyzed Hydration4. Acid-catalyzed Hydration• The presence of the large bulky group

(OSO3H) of the alkyl hydrogensulphate makes it very unstable

• Two possible further reactions may take place

1. Regeneration of 1. Regeneration of AlkenesAlkenes

• On heating, alkyl hydrogensulphates form alkenes and sulphuric acid

2. Production of 2. Production of AlcoholsAlcohols• Alkyl hydrogensulphates can be easily

hydrolyzed to alcohols by heating with water

Catalytic Catalytic HydrogenationHydrogenation• Alkenes react with hydrogen in the

presence of metal catalysts (e.g. Ni, Pd, Pt) to give alkanes

Catalytic Catalytic HydrogenationHydrogenation

Catalytic Catalytic HydrogenationHydrogenation• Useful in analyzing unsaturated

hydrocarbons

• Fats and oils are organic compounds called triglycerides

esters formed from glycerol and carboxylic acids of long carbon

chains

• Fats and oils are either saturated or unsaturated

• Saturated fats

solids at room temp

usually come from animal sources

• Unsaturated fats

liquids at room temp

primarily derived from plants

Catalytic Catalytic HydrogenationHydrogenation• Fats and oils are similar in structure

• Only difference is the presence of C=C double bonds in the acid components of oils

lower their m.p.

make them liquids at room temp

• Fats are stable towards oxidation by air

• More convenient to handle and store

Catalytic Catalytic HydrogenationHydrogenation• Can be employed to convert the C=C

double bonds present in oils to saturated fats (i.e. margarine)

• The conversion is also known as hardening of oils

• Advantage:

turning rancid much less readily than unsaturated oils

Hydrogenation of vegetable oils produces margarine

Check Point 28-5BCheck Point 28-5B

Relative Stability of Alkenes in Relative Stability of Alkenes in Terms of Enthalpy Changes of Terms of Enthalpy Changes of HydrogenationHydrogenation

• Hydrogenation of alkenes is exothermic

• From enthalpy changes of hydrogenation

predict the relative stabilities of alkenes

Enthalpy changes of hydrogenation of but-1-ene, cis-but-2-ene and trans-but-2-ene

28.5 Reactions of Alkenes (SB p.184)Relative Stability of Alkenes in Relative Stability of Alkenes in Terms of Enthalpy Changes of Terms of Enthalpy Changes of HydrogenationHydrogenation

• The pattern of the relative stabilities of alkenes determined from the enthalpy changes of hydrogenation:

Check Point 28-5CCheck Point 28-5C

The END

Give the IUPAC names for the following alkenes:

Answer(a) trans-3,4-Dichlorohept-3-ene

Give the IUPAC names for the following alkenes:

Answer(b) cis-3,4-Dimethyloct-3-ene

Draw the structural formula for each of the following alkenes:

(a) cis-Hex-3-ene

(b) trans-2,3-Dihydroxybut-2-ene

(c) cis-1,2-DichloroetheneAnswer

(a) (c)

Classify the following alcohols as primary, secondary or tertiary alcohols.

(a) CH3CHOHCH2CH3

(b) CH3CH2CH2OH

(c) (CH3)2COHCH2CH2CH3 Answer(a) It is a secondary alcohol.

(b) It is a primary alcohol.

(c) It is a tertiary alcohol. Back

Classify the following haloalkanes as primary, secondary or tertiary haloalkanes.

(a) (c)

Answer

(a) A secondary haloalkane

(b) A primary haloalkane

(c) A tertiary haloalkane

Back28.4 Preparation of Alkenes (SB p.173)

Of the isomeric C5H11+ carbocations, which one

is the most stable?Answer

The more stable C5H11+ carbocation is the tertiary

carbocation as shown below:

Both alkanes and alkenes undergo halogenation. The

halogenation of alkanes is a free radical substitution

reaction while the reaction of alkenes with halogens is

an electrophilic addition reaction. Can you tell two differences between the products formed by the

twodifferent types of halogenation?

AnswerAlkenes give dihalogenated products while alkanes usually give

polysubstituted products. Another difference is the position of the

attachment of the halogen atom. For alkenes, the halogen atom is

fixed to the carbon atom of the carbon=carbon double bond. In the

substitution reaction of alkanes, the position of of the halogen atom

varies.

(a) What chemical tests would you use to distinguish between two unlabelled bottles containing hexane and hex-1-ene respectively? Answer

(a) We can perform either one of the following tests:

Hex-1-ene can decolourize bromine water or chlorine water in

the dark while hexane cannot.

Hex-1-ene can decolourize acidified potassium manganate(VII)

solution while hexane cannot.

(b) What is the major product of each of the following reactions?

Answer

(b) (i)

(c) Give the products for the following reactions:

(i) CH3CH = CH2 + H2

(ii) CH3CH = CHCH3

(iii) CH3CH = CHCH3 + Br2

conc. H2SO4

Answer

(c) (i) CH3CH2CH3

(a) Arrange the following carbocations in increasing order of stability. Explain your answer briefly.

Answer

(a) The increasing order of the stability of carbocations is:

Tertiary carbocations are the most stable because the three alkyl

groups release electrons to the positive carbon atom and thereby

disperse its charge. Primary carbocations are the least stable as t

here is only one alkyl group releasing electrons to the positive car

bon atom.

(b) Based on your answer in (a), arrange the following molecules in the order of increasing rates of reaction with hydrogen chloride.

Answer

(b) The reaction of these compounds with hydrogen chloride involves

the formation of carbocations. Therefore, the order of reaction rate

s follows the order of the ease of the formation of carbocations, i.e.

the stability of carbocations:

Therefore, the rates of reactions of the three compounds with hydr

ogen chloride increase in the order:

Alkenes and Electrophilic Addition

positively

level book

highly substituted

positive carbon

concentrated

catalytic

hydrogen halide

electrophilic

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