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Alkenes and Alkynes
Alkenes are compounds that contain a C=C bond that reacts with numerous
electrophiles in electrophilic addition reactions,
and in pericyclic reactions
• Double bond is formed by one sigma and
one pi bond, the sigma bond occurs with
the end on overlap of two C sp2
hybridized orbital, the pi bond forms
when two unhybridized p orbitals overlap
side on, the three sigma bonds formed by
the central carbon (two to other atoms)
are in the same plane and separated by angles of 120 degrees
Alkynes contain a triple C-‐C bond, also can react with electrophiles but less
reactive
• Triple bond formed of one sigma and
two pi-‐ sigma bond formed of end-‐on
overlap of sp hybridized C orbitals, the
pi bonds formed by the side-‐on overlap
of two unhybridized p orbitals on each C
atom, the two sigma bonds on C (one to
another atom) are separated by an angle of 180 degrees
• To name, select the longest continuous chain containing the alkyne, use
the suffix yne and number the chain to include both atoms of the triple
bond with the lowest number for the first carbon of the alkyne,
4-methyl-1,5-octadiyne
Stability of C=C bonds
• Rotation around C=C bond restricted because requires pi bond can break-‐
gives rise to different configurational isomers-‐ E and Z
• The more stable configurational isomers have the two largest
substituents on the opposite sides of the double bond because this
minimizes steric strain-‐ E often easiest to make as more stable
• The more alkyl groups attached to a C=C bond the more stable it is-‐ not
well understood but one explanation is that the pair of electrons in a filled
C-‐H σ orbital interacts with the empty π* in the C=C-‐ leads to the two
electrons becoming more delocalized which leads to more stability-‐ this is
hyperconjugation
Electrophilic addition
• C=C and alkyne bonds are electron rich so can act as nucleophiles in
reactions-‐ electrophilic additions, general reaction:
Reactivity of the bonds towards electrophiles depends on the steric and
electronic effects of the substitutes:
• The more alkyl groups attached to the bond, the faster the rate of
reaction as the alkyl groups are +I they make the bond more
nucleophilic and more likely to react to electrophiles
• -‐I/ -‐M groups make the CC bond less nucleophilic and less reactive to
nucleophiles
Preparation of Alkenes
• Halogenoalkanes react with bases in E1/E2 reactions to form a range of
alkenes
• Alcohols undergo elimination reactions when using reagents that convert
the OH group of the alcohol into a compound that provides a good leaving
group – e
...
heating with an acid can lead to the loss of a water molecule-‐
dehydration-‐ in an E1 reaction to form
an alkene
...
If a platinum catalyst is used, addition to both alkyne/ C=C bonds
are fast and the alkyne is converted to an alkane-‐ hydrogenation-‐ uses
heterogeneous catalysis-‐ forms z alkene
• Can be done by partial reduction using sodium in liquid ammonia to form
E alkenes-‐ sodium metal dissolves in ammonia and forms Na+-‐ add to
alkyne bonds to form
radical anions-‐
protonated by
ammonia to form a
vinyl radical which
accepts and electron
from the Na to form a
vinyl anion, again
protonated by
ammonia to form E alkene (minimizes steric strain)
• Wittig reaction: a phosphorone (phosphonium ylide, Ph3P=CR2/CHR/CH2)
reacts with an aldehyde or ketone-‐
replaces the oxygen atom in the
aldehyde/ketone with the
CR2/CHR/CH2 group
Preparation of Alkynes
• One mole of 1,2-‐dibromoalkanes (vicinal dibromides) is reacted with
2 moles of a strong base, e
...
sodium amide (NaNH2)
• Mechanisms involve two consecutive E2 reactions that lead to the
elimination of two molecules of HBr from the 1,2-‐dibromoalkane-‐ the
C-‐H and C-‐Br are anti-‐periplanar
•
Also:
Substituted alyknes are also formed from the reaction of alkynyl
anions with halogenoalkanes in nucleophilic substitution reactions-‐
alkynyl anions are strong nucleophiles that react with various primary
halogenoalkanes in SN2 reactions to form a new C-‐C bond
Addition of hydrogen halides
• Alkenes react with HX in two step
reaction to produce
halogenoalkanes
• In the first slower step of the
reaction, the two electrons in the
C=C pi bond (nucleophile) are
attracted towards the partially positive H in HX-‐ HX bond breaks
• New C-‐H bond forms and two electrons in HX move on to the halogen
atom-‐ produces intermediate planar carbocation and halide ion which
then rapidly reacts with carbocation to produce the halogenoalkane
• Addition leads to a decrease in entropy but thermodynamically favored
because of enthalpy change-‐ large negative value, additions exothermic
because CH/ CX bonds formed are much stronger than those broken
Markonikov
• When HX adds to unsymmetrical alkenes-‐ different groups
around C=C-‐ reactions may be regioselective-‐ the H and X
atoms can add to the C atoms so as to selectively form the more
substituted halogenoalkane-‐ known as the Markonikov product
• Markonikov rule states the following: on addition of HX to an
alkene, H attaches to the carbon with the fewest alkyl groups
and X to the C with the most alkyl groups
• Secondary carbocation is selectively formed over primary
because more stable-‐ two +I alkyl groups-‐ reaction to form
more stable carbocation has the lower Gibbs energy of
activation
Rearrangement of intermediate carbocations
• Intermediate carbocations formed in addition reactions can undergo
rearrangements to form more stable carbocations-‐ involving movement
of a H atom or alkyl group
• When a hydrogen atom moves from a C atom next to the carbocation to
the positively charged C, this is called a 1,2-‐hybride shift-‐ H moves with a
pair of electrons (H-‐ moves) and 1,2 is used to show moves to adjacent
carbon-‐ can result in different major and minor products forming
•
When an alkyl group moves from a C next to the carbocation to the
positively charged carbon, this is called a 1,2-‐alkyl shift (Wagner-‐
Meerwein rearrangement-‐ the alkyl groups with a pair of electrons
to form a more stable carbocation
Addition of bromine
• Bromine adds to the C=C bond of alkenes to form 1,2-‐
dirbomoalkanes in electrophilic addition reactions
•
•
•
•
•
•
•
Br2 becomes polarized when approaching the C=C bond, the
electrons are repelled by the electron rich C=C bond and move
away from the Br atom that is nearer the bond-‐ makes Br atom
nearest the bond electrophilic-‐ slightly positively charged
The C=C can act as a nucleophile and donates a pair of electrons to
the polarized Br molecule to form C-‐Br and Br-‐
The partially positive Br atom donates a lone pair of electrons to
one of the carbon atoms to form a second C-‐Br bond-‐ bromine
atom simultaneously bonds to both carbons to make an
intermediate 3 membered ring called a bromium ion
The positively charged bromine is a strong –I group so electrons in
two C-‐Br bonds are attracted away from both C atoms-‐ C atoms in
the ring electrophilic-‐ formation of ion reversible process
Br-‐ acts as the nucleophile and rapidly reacts with the bromonium
ion in an SN2 reaction-‐ Br-‐ approaches the bromonium ion from
opposite side to positively charged bromine
...
)
Br2 molecule can react with either top or bottom face of C=C bond
and as the Br-‐ can react with either carbon atom, the ion is formed
as a racemate (1:1 mixture of enantiomers) which means that the
dibromide product is also a racemate
Evidence for the mechanism for addition of bromine to a C=C bond
• Provided by the reaction of cyclic alkenes with bromine to produce cyclic
1,2-‐dibromides-‐ rotation around the
BrC-‐CBr bond is not possible in
these compounds so the relative
positions of the two bromine in the
products are fixed
• Different stereoisomers of the
reactant produce different
stereoisomers of the product
• Addition of Br2 to the E isomer forms a meso compound-‐ has plane of
symmetry-‐ so achiral
• Addition of Br2 to the Z isomer forms a racemate
• The meso compound
and the racemate
are diastereomers
(different
configuration at one
of the two chiral
centres)
Addition of bromine in the presence of water
• When water is present in the
reaction of Br2 with a C=C bond, a
1,2-‐bromoalcohol (bromohydrin)
is formed
• Mechanism similar but water acts
as the nucleophile in the second
step
• If both Br-‐ and H2O acted as a
nucleophile would produce a mixture of bromohydrin and 1,2-‐
dibromide
• Br-‐ stronger nucleophile but major product depends on the
concentration of the nucleophile as this increases the rate of the
ring opening
• Reaction with water is regioselective-‐ will attack the more
substituted C in the bromonium ion-‐ this atom form a longer and
weaker bond to the Br because better at stabilizing the positive
charge (overcomes fact that sterically hindered)
• In the transition state for ring opening of the bromonium ion,
breaking of the C-‐Br bond occurs to a greater extent than
formation of the new C-‐O bond-‐ described as a loose SN2 transition
state
Hydration reactions
• Alkenes react with water in the presence of a strong acid to form alcohols-‐
water adds to the C=C bond
• The C=C bond is protonated to form a carbocation-‐ regioselective for
unsymmetrical alkenes-‐ tertiary carbocation formed more readily-‐ leads
to selectively formed Markovnikov product
• A nucleophile then reacts with the carbocation-‐ nucleophile can be water
or the conjugate base of the acid-‐ nature depends on concentration of the
acid-‐ when dilute water reacts, when concentrated conjugate base reacts
• Oxonium ion is formed and water reacts with this to form an alcohol
• All steps
reversible-‐
dehydration
possible
Addition of borane followed by oxidation
• Alcohols can be formed from alkenes by reaction of the C=C bond with
diborane (B2H6) followed by oxidation using hydrogen peroxide (H2O2)
and aqueous sodium hydroxide
...
Hydroxide ion
acts as leaving group-‐ poor leaving group but migration of alkyl group
occurs because CO bond replaces weaker OO bond-‐ boron atom becomes
planar once more
4) Trialkylborate (RO)3B reacts with hydroxide ion and water in a series of
three nucleophilic substitution reactions to form three molecules of the
alcohol and boric acid ((HO)3B)
•
The anti-‐Markovnikov product is the one that's formed-‐ conversion of CB
to CO bonds stereospecific-‐ alkyl groups move with retention of
configuration-‐ hydroboration followed by oxidation results in syn
addition of H and OH groups to the C=C bond
Reaction with peroxycarbolic acids
• Peroxycarbolic acids have an oxygen atom between the C=O and OH
groups of a carboxylic acid-‐ react with alkenes by donating an oxygen
atom to a C=C bond to form an epoxide-‐ called epoxidation-‐ electrophilic
addition
• Oxygen atom in OH group is partially positive because RCO2 group –I and
acts as the electrophile for C=C-‐ weak OO bond breaks
• Pair of electrons moves on to carbonyl group and proton abstracted from
OH group at the same time a second CO bond forms
• Because epoxidation takes place in a single step, the CC bond of the alkene
cannot rotate and change from trans to cis configuration during the
addition-‐ explains why epoxide retains configuration of C=C bond-‐
reaction stereospecific-‐ peroxycarboxylic acid can approach either side of
the C=C bond equally well so the epoxide is formed as a racemate
Reactions of epoxides
• Three membered epoxide ring is highly strained and can be opened by
reaction with a wide range of nucleophiles
Acid-‐catalysed ring opening
• Water weak nucleophile but will react with
protonated epoxide as it becomes stronger
electrophile
• SN2 reaction in which the water molecule
approaches the epoxide to one of the CO bonds-‐
substitution produces a new CO bond at the
same time as one is broken
• Deprotonation of oxonium ion forms 1,2-‐diol
that has the two OH groups in an anti
arrangement-‐ H3O+ is catalyst as regenerated
Alkali-‐catalysed ring opening
• Hydroxide ion stronger nucleophile-‐ reacts in SN2 reaction
• Hydroxide ion approaches epoxide from opposite side to CO bonds
• Protonation of ion forms 1,2-‐diol
Pericyclic reactions of alkenes
• Single-‐step (concerted) reactions involving change in the position of
bonding electrons via cyclic transition states
• Most common involving C=C bonds are cycloaddition-‐ C=C bond of
alkene reacts with another molecule to form a cyclic product
Potassium permanganate KMnO4
• Used to convert an alkene into a 1,2-‐diol-‐ dihydroxylation reaction
• Cold, dilute solution KMnO4 reacts with C=C bond in pericyclic
reaction to form manganate ester
• C=C nucleophile attacks oxygen atom on manganate ion to form CO
bond-‐ pair of electrons moves on to manganese and another pair in
adjacent Mn=O bond moves to second CO bind-‐ both bonds formed
on same face-‐ syn reaction
• Usually carried out in the presence of aq hydroxide so manganate
ester is immediately hydrolysed to form a 1,2-‐diol
• As single step
configuration
retained-‐
stereospecific-‐
top or bottom
face of C=C
reacted with the same-‐ 1,2-‐diol racemate
• Reaction must be kept cold because KMnO4 is such a powerful
oxidising agent, that at room temperature or above it oxidizes the
alcohols to form ketones and carboxylic acids-‐ oxidative cleavage
Osmium tetroxide OsO4
• Reacts in same pericyclic reaction as above to
form osmate ester-‐ same stereochem
• Usually carried out in the presence of water
so osmate ester immediately hydrolysed
• Osmium tetroxide is expensive and toxic so
to reduce amounts oxidizing agent is added
to mixture so as soon as it is converted into
Os(OH)2O2 the oxidizing agent selectively
oxidizes it back to OsO4
• Potassium ferricyanide, periodic acid (also
oxidizes alcohol to carbonyls-‐ cleaves CC
bond via oxidative cleavage)
Reaction of alkenes with ozone
• O3 (dipolar reagent-‐ both negative and positive charge) used to convert
C=C bond into two C=O bonds, forming aldehydes/ ketones/ carboxylic
acids depending on the structure of the alkene and whether an oxidizing
or reducing agent is used
• It reacts with C=C bond in concerted cycloaddition reaction to form
molozonide-‐ unstable due to weak O-‐O bonds
• The bonds break to form a carbonyl oxide and an aldehyde which must
flip over to react with the carbonyl ocide in 1,3 dipolar cycloaddition to
form an oxonide
• The oxonide then can be reduced or oxidized into different carbonyl
products
Electrophilic addition reactions of alkynes-‐ electron rich alkyne bond acts as
a nucleophile
Hydrogen halide addition
• React with HX in two step reactions, same mechanism with alkenes, to
form halogenoalkenes
• Reaction slower as addition of proton forms vinyl cation which is less
stable than an alkyl cation-‐ the greater the s character of the hybrid
orbital, the closer the electrons are held to the nucleus and the more
electron withdrawing-‐ sp3 carbon of alkyl substituent has less s
character than sp2 carbon of alkene-‐ cannot stabilize adjacent positive
charge as well as an alkyl group-‐ Gibbs activation for formation higher
• Addition of HX to a C=C bond can often be stopped after addition of
one equivalent of HX-‐ if two equivalents react with a C=C bond, a
second molecule of HX adds to the halogenoalkane to form germinal
halogenoalkane (both halogens on same carbon)
Dw;s
Addition of borane to alkynes followed by oxidation
• Aldehydes/ketones formed from alkynes by reaction of the C=C bond
with diborane followed by oxidation using hydrogen peroxide/ sodium
hydroxide
• Terminal alkyne RCCH
reacts for form
aldehyde, internal
alkyne RCCR reacts to
form ketone
• Mechanism similar to
that of addition of an
alkene but stage 3
produces an enol that
rearranges to form an
aldehyde
• The equilibrium
between an aldehyde/
ketone and an enol is
called tautomerism
Diels-‐Alder Reaction
•
•
The [4+2]-‐cycloaddition of a conjugated diene and a dienophile (an
alkene or alkyne), an electrocyclic reaction that involves the 4 π-‐electrons
of the diene and 2 π-‐electrons of the dienophile
...
A variant is the hetero-‐Diels-‐Alder, in which either the diene or the
dienophile contains a heteroatom, most often nitrogen or oxygen
...
Overlap of the molecular orbitals is
required:
Overlap between the highest occupied
MO of the diene (HOMO) and the lowest
unoccupied MO of the dienophile
(LUMO) is thermally allowed in the
Diels Alder Reaction, provided the
orbitals are of similar energy
...
Good dienophiles often bear one or
two of the following substituents: CHO, COR, COOR, CN, C=C, Ph, or halogen
...
The
reaction is diastereoselective
...
The endo product is
usually favored by kinetic
control due to secondary orbital
interactions
...
, the more stable alkene (more highly substituted
alkene) is formed as the major product
...
,) then the less
substituted alkene product predominates
...
2) Use of single headed ‘fishhook’ arrows denotes the movement of
1 electron
...
4) Radicals are electron deficient so they are stabilised by the same things that stabilise carbocations