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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
A-Level Book 3A A-Level 3A2
28.28.11IntroductionIntroduction
A-Level Book 3A A-Level 3A3
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
A-Level Book 3A A-Level 3A4
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
A-Level Book 3A A-Level 3A5
28.1 Introduction (SB p.167)
AlkeneAlkeness bond can be broken down easily
alkenes are reactive compounds
undergo mainly addition reactions
A-Level Book 3A A-Level 3A6
28.1 Introduction (SB p.167)
AlkeneAlkeness• Limited rotation of the C=C double bond
alkenes show geometrical isomerism
• e.g.
A-Level Book 3A A-Level 3A7
28.28.22NomenclaturNomenclature of Alkenese of Alkenes
A-Level Book 3A A-Level 3A8
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”
A-Level Book 3A A-Level 3A9
28.2 Nomenclature of Alkenes (SB p.167)
IUPAC Rules of Naming of IUPAC Rules of Naming of AlkenesAlkenes
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
A-Level Book 3A A-Level 3A10
28.2 Nomenclature of Alkenes (SB p.168)
IUPAC Rules of Naming of IUPAC Rules of Naming of AlkenesAlkenes
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
A-Level Book 3A A-Level 3A11
28.2 Nomenclature of Alkenes (SB p.168)
IUPAC Rules of Naming of IUPAC Rules of Naming of AlkenesAlkenese.g.
A-Level Book 3A A-Level 3A12
28.2 Nomenclature of Alkenes (SB p.168)
IUPAC Rules of Naming of IUPAC Rules of Naming of AlkenesAlkenes
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
A-Level Book 3A A-Level 3A13
28.2 Nomenclature of Alkenes (SB p.168)
IUPAC Rules of Naming of IUPAC Rules of Naming of AlkenesAlkenese.g.
Check Point 28-2Check Point 28-2Example 28-2Example 28-2
A-Level Book 3A A-Level 3A14
28.28.33 Physical Physical Properties of Properties of
AlkenesAlkenes
A-Level Book 3A A-Level 3A15
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
A-Level Book 3A A-Level 3A16
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
A-Level Book 3A A-Level 3A17
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
A-Level Book 3A A-Level 3A18
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
A-Level Book 3A A-Level 3A19
28.28.44Preparation Preparation
of Alkenesof Alkenes
A-Level Book 3A A-Level 3A20
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
A-Level Book 3A A-Level 3A21
28.4 Preparation of Alkenes (SB p.170)
CrackinCrackingg
e.g.
2CH3CH3 CH2 = CH2 + 2CH4600 oC
2CH3CH2CH3
CH3CH = CH2 + CH2 = CH2 + CH4 + H2
600 oC
A-Level Book 3A A-Level 3A22
28.4 Preparation of Alkenes (SB p.170)
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
A-Level Book 3A A-Level 3A23
28.4 Preparation of Alkenes (SB p.170)
1. Dehyhalogenation
1. Dehyhalogenation• Elimination of a hydrogen halide
molecule from a haloalkane
• By heating the haloalkane in an alcoholic solution of KOH
A-Level Book 3A A-Level 3A24
28.4 Preparation of Alkenes (SB p.170)
1. Dehyhalogenation
1. Dehyhalogenatione.g.
A-Level Book 3A A-Level 3A25
28.4 Preparation of Alkenes (SB p.171)
1. Dehyhalogenation
1. Dehyhalogenation• The ease of dehydrohalogenation of
haloalkanes decreases in the order:
Tertiary haloalkane
Secondary haloalkane
>
Primaryhaloalkane
>
A-Level Book 3A A-Level 3A26
28.4 Preparation of Alkenes (SB p.171)
1. Dehyhalogenation
1. Dehyhalogenation• Different classes of haloalkanes or
alcohols have different reactivities
• May undergo different types of reactions under the same reaction conditions
A-Level Book 3A A-Level 3A27
28.4 Preparation of Alkenes (SB p.171)
1. Dehyhalogenation
1. Dehyhalogenation• Dehydrohalogenation of secondary or
tertiary haloalkanes can take place in more than one way
• A mixture of alkenes is formed
A-Level Book 3A A-Level 3A28
28.4 Preparation of Alkenes (SB p.171)
1. Dehyhalogenation
1. Dehyhalogenation
CH3CH2CHClCH3
CH3CH=CHCH3 + CH3CH2CH=CH2
2-Chlorobutane
But-2-ene (80 %) But-1-ene (20 %)
alc. KOH
heat
e.g.
• The more highly substituted alkene (i.e. but-2-ene) is the major product
A-Level Book 3A A-Level 3A29
28.4 Preparation of Alkenes (SB p.172)
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
A-Level Book 3A A-Level 3A30
28.4 Preparation of Alkenes (SB p.172)
1. Dehyhalogenation
1. Dehyhalogenation• The relative stabilities of alkenes
decrease in the order:
A-Level Book 3A A-Level 3A31
28.4 Preparation of Alkenes (SB p.172)
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
A-Level Book 3A A-Level 3A32
28.4 Preparation of Alkenes (SB p.172)
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
A-Level Book 3A A-Level 3A33
28.4 Preparation of Alkenes (SB p.172)
2. Dehydration of Alcohols2. Dehydration of Alcohols
• Primary alcohols generally required concentrated sulphuric acid and a relatively high temperature
A-Level Book 3A A-Level 3A34
28.4 Preparation of Alkenes (SB p.172)
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)
A-Level Book 3A A-Level 3A35
28.4 Preparation of Alkenes (SB p.173)
2. Dehydration of Alcohols2. Dehydration of Alcohols
• The relative ease of dehydration of alcohols generally decreases in the order:
Tertiary alcohol
Secondary alcohol
>
Primaryalcohol
>
A-Level Book 3A A-Level 3A36
28.4 Preparation of Alkenes (SB p.173)
2. Dehydration of Alcohols2. Dehydration of Alcohols
• 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
A-Level Book 3A A-Level 3A37
28.4 Preparation of Alkenes (SB p.174)
Addition Addition ReactionsReactionsHydrogenationHydrogenation• Alkenes can be prepared by hydrogenation
of alkynes
Depend on the conditions and the catalyst employed
A-Level Book 3A A-Level 3A38
28.4 Preparation of Alkenes (SB p.174)
HydrogenationHydrogenation
• Lindlar’s catalyst is metallic palladium deposited on calcium carbonate
further hydrogenation of the alkenes formed can be prevented
A-Level Book 3A A-Level 3A39
28.528.5Reactions of Reactions of
AlkenesAlkenes
A-Level Book 3A A-Level 3A40
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
A-Level Book 3A A-Level 3A41
28.5 Reactions of Alkenes (SB p.174)
Why do Alkenes Undergo Addition Why do Alkenes Undergo Addition Reactions Readily?Reactions Readily?
• 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
A-Level Book 3A A-Level 3A42
28.5 Reactions of Alkenes (SB p.174)
Why do Alkenes Undergo Addition Why do Alkenes Undergo Addition Reactions Readily?Reactions Readily?
A-Level Book 3A A-Level 3A43
28.5 Reactions of Alkenes (SB p.174)
Why do Alkenes Undergo Addition Why do Alkenes Undergo Addition Reactions Readily?Reactions Readily?
• The electrons of the bond are
diffuse in shape
less firmly held by the bonding carbon
nuclei
• Susceptible to the attack by electrophiles
A-Level Book 3A A-Level 3A44
28.5 Reactions of Alkenes (SB p.175)
Why do Alkenes Undergo Addition Why do Alkenes Undergo Addition Reactions Readily?Reactions Readily?
• Electrophiles that attack the C=C double bond
protons (H+)
neutral species in which the molecule is polarized, e.g. bromine
A-Level Book 3A A-Level 3A45
28.5 Reactions of Alkenes (SB p.175)
Electrophilic Addition Electrophilic Addition ReactionsReactions• Addition of electrophiles to the C=C
double bond of alkenes
A-Level Book 3A A-Level 3A46
28.5 Reactions of Alkenes (SB p.175)
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
A-Level Book 3A A-Level 3A47
28.5 Reactions of Alkenes (SB p.175)
1. Addition of Hydrogen Bromide1. Addition of Hydrogen Bromidee.g. the addition of HBr to
ethene produces bromoethane
A-Level Book 3A A-Level 3A48
28.5 Reactions of Alkenes (SB p.175)
1. Addition of Hydrogen Bromide1. Addition of Hydrogen Bromide• When but-2-ene reacts with HBr
2-bromobutane is formed as the only product
A-Level Book 3A A-Level 3A49
28.5 Reactions of Alkenes (SB p.175)
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
A-Level Book 3A A-Level 3A50
28.5 Reactions of Alkenes (SB p.176)
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
A-Level Book 3A A-Level 3A51
28.5 Reactions of Alkenes (SB p.176)
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
A-Level Book 3A A-Level 3A52
28.5 Reactions of Alkenes (SB p.176)
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-Level Book 3A A-Level 3A53
28.5 Reactions of Alkenes (SB p.176)
Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
• 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”
A-Level Book 3A A-Level 3A54
28.5 Reactions of Alkenes (SB p.176)
Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
A-Level Book 3A A-Level 3A55
28.5 Reactions of Alkenes (SB p.177)
Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
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
A-Level Book 3A A-Level 3A56
28.5 Reactions of Alkenes (SB p.177)
Regioselectivity of Hydrogen Halide Regioselectivity of Hydrogen Halide Addition: Markovnikov’s RuleAddition: Markovnikov’s Rule
• The products formed according to this rule are known as Markovnikov products
A-Level Book 3A A-Level 3A57
28.5 Reactions of Alkenes (SB p.177)
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
A-Level Book 3A A-Level 3A58
28.5 Reactions of Alkenes (SB p.178)
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
A-Level Book 3A A-Level 3A59
28.5 Reactions of Alkenes (SB p.178)
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:
A-Level Book 3A A-Level 3A60
28.5 Reactions of Alkenes (SB p.178)
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
A-Level Book 3A A-Level 3A61
28.5 Reactions of Alkenes (SB p.178)
Check Point 28-5ACheck Point 28-5A
A-Level Book 3A A-Level 3A62
28.5 Reactions of Alkenes (SB p.178)
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:
A-Level Book 3A A-Level 3A63
28.5 Reactions of Alkenes (SB p.178)
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:
A-Level Book 3A A-Level 3A64
28.5 Reactions of Alkenes (SB p.179)
2. Addition of Halogens2. Addition of Halogens• Halogens normally react with alkenes by
electrophilic addition
where X2 can be F2, Cl2 or Br2
A-Level Book 3A A-Level 3A65
28.5 Reactions of Alkenes (SB p.179)
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)
A-Level Book 3A A-Level 3A66
28.5 Reactions of Alkenes (SB p.179)
2. Addition of Halogens2. Addition of Halogens
A-Level Book 3A A-Level 3A67
28.5 Reactions of Alkenes (SB p.179)
2. Addition of Halogens2. Addition of Halogense.g.
A-Level Book 3A A-Level 3A68
28.5 Reactions of Alkenes (SB p.180)
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
d
A-Level Book 3A A-Level 3A69
28.5 Reactions of Alkenes (SB p.180)
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
A-Level Book 3A A-Level 3A70
28.5 Reactions of Alkenes (SB p.180)
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
A-Level Book 3A A-Level 3A71
28.5 Reactions of Alkenes (SB p.180)
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
A-Level Book 3A A-Level 3A72
28.5 Reactions of Alkenes (SB p.181)
4. Acid-catalyzed Hydration4. Acid-catalyzed Hydration
• Alkenes dissolve in cold and concentrated sulphuric acid
A-Level Book 3A A-Level 3A73
28.5 Reactions of Alkenes (SB p.181)
4. Acid-catalyzed Hydration4. Acid-catalyzed Hydratione.g.
A-Level Book 3A A-Level 3A74
28.5 Reactions of Alkenes (SB p.181)
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
A-Level Book 3A A-Level 3A75
28.5 Reactions of Alkenes (SB p.181)
1. Regeneration of 1. Regeneration of AlkenesAlkenes
• On heating, alkyl hydrogensulphates form alkenes and sulphuric acid
A-Level Book 3A A-Level 3A76
28.5 Reactions of Alkenes (SB p.181)
2. Production of 2. Production of AlcoholsAlcohols• Alkyl hydrogensulphates can be easily
hydrolyzed to alcohols by heating with water
A-Level Book 3A A-Level 3A77
28.5 Reactions of Alkenes (SB p.182)
Catalytic Catalytic HydrogenationHydrogenation• Alkenes react with hydrogen in the
presence of metal catalysts (e.g. Ni, Pd, Pt) to give alkanes
A-Level Book 3A A-Level 3A78
28.5 Reactions of Alkenes (SB p.182)
Catalytic Catalytic HydrogenationHydrogenation
e.g.
A-Level Book 3A A-Level 3A79
28.5 Reactions of Alkenes (SB p.182)
Catalytic Catalytic HydrogenationHydrogenation• Useful in analyzing unsaturated
hydrocarbons
A-Level Book 3A A-Level 3A80
28.5 Reactions of Alkenes (SB p.182)
Catalytic Catalytic HydrogenationHydrogenation
• 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
A-Level Book 3A A-Level 3A81
28.5 Reactions of Alkenes (SB p.182)
Catalytic Catalytic HydrogenationHydrogenation
• Saturated fats
solids at room temp
usually come from animal sources
A-Level Book 3A A-Level 3A82
28.5 Reactions of Alkenes (SB p.182)
Catalytic Catalytic HydrogenationHydrogenation
• Unsaturated fats
liquids at room temp
primarily derived from plants
A-Level Book 3A A-Level 3A83
28.5 Reactions of Alkenes (SB p.182)
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
A-Level Book 3A A-Level 3A84
28.5 Reactions of Alkenes (SB p.182)
Catalytic Catalytic HydrogenationHydrogenation
• Fats are stable towards oxidation by air
• More convenient to handle and store
A-Level Book 3A A-Level 3A85
28.5 Reactions of Alkenes (SB p.182)
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
A-Level Book 3A A-Level 3A86
28.5 Reactions of Alkenes (SB p.182)
Catalytic Catalytic HydrogenationHydrogenation
Hydrogenation of vegetable oils produces margarine
Check Point 28-5BCheck Point 28-5B
A-Level Book 3A A-Level 3A87
28.5 Reactions of Alkenes (SB p.183)
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
A-Level Book 3A A-Level 3A88
28.5 Reactions of Alkenes (SB p.183)
Enthalpy changes of hydrogenation of but-1-ene, cis-but-2-ene and trans-but-2-ene
A-Level Book 3A A-Level 3A89
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:
A-Level Book 3A A-Level 3A90
Check Point 28-5CCheck Point 28-5C
28.5 Reactions of Alkenes (SB p.184)
A-Level Book 3A A-Level 3A91
The END
A-Level Book 3A A-Level 3A92
28.2 Nomenclature of Alkenes (SB p.168)
Give the IUPAC names for the following alkenes:
(a)
Answer(a) trans-3,4-Dichlorohept-3-ene
A-Level Book 3A A-Level 3A93
28.2 Nomenclature of Alkenes (SB p.168)
Give the IUPAC names for the following alkenes:
(b)
Answer(b) cis-3,4-Dimethyloct-3-ene
Back
A-Level Book 3A A-Level 3A94
28.2 Nomenclature of Alkenes (SB p.169)
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)
(b)
Back
A-Level Book 3A A-Level 3A95
28.4 Preparation of Alkenes (SB p.173)
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
A-Level Book 3A A-Level 3A96
Classify the following haloalkanes as primary, secondary or tertiary haloalkanes.
(a) (c)
(b)
Answer
(a) A secondary haloalkane
(b) A primary haloalkane
(c) A tertiary haloalkane
Back28.4 Preparation of Alkenes (SB p.173)
A-Level Book 3A A-Level 3A97
28.5 Reactions of Alkenes (SB p.177)
Of the isomeric C5H11+ carbocations, which one
is the most stable?Answer
The more stable C5H11+ carbocation is the tertiary
carbocation as shown below:
Back
A-Level Book 3A A-Level 3A98
28.5 Reactions of Alkenes (SB p.179)
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?
Back
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-Level Book 3A A-Level 3A99
28.5 Reactions of Alkenes (SB p.183)
(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.
A-Level Book 3A A-Level 3A100
28.5 Reactions of Alkenes (SB p.183)
(b) What is the major product of each of the following reactions?
(i)
(ii)
Answer
A-Level Book 3A A-Level 3A101
28.5 Reactions of Alkenes (SB p.183)
(b) (i)
(ii)
A-Level Book 3A A-Level 3A102
(c) Give the products for the following reactions:
(i) CH3CH = CH2 + H2
(ii) CH3CH = CHCH3
(iii) CH3CH = CHCH3 + Br2
Ni
conc. H2SO4
28.5 Reactions of Alkenes (SB p.183)
Answer
A-Level Book 3A A-Level 3A103
28.5 Reactions of Alkenes (SB p.183)
(c) (i) CH3CH2CH3
(ii)
(iii)
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A-Level Book 3A A-Level 3A104
28.5 Reactions of Alkenes (SB p.184)
(a) Arrange the following carbocations in increasing order of stability. Explain your answer briefly.
Answer
A-Level Book 3A A-Level 3A105
28.5 Reactions of Alkenes (SB p.184)
(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.
A-Level Book 3A A-Level 3A106
28.5 Reactions of Alkenes (SB p.184)
(b) Based on your answer in (a), arrange the following molecules in the order of increasing rates of reaction with hydrogen chloride.
Answer
A-Level Book 3A A-Level 3A107
28.5 Reactions of Alkenes (SB p.184)
(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:
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