Transcript ADDITION REACTIONS

ADDITION REACTIONS
REACTIONS OF ALKENES
• A reaction in which the double bond of an
alkene is converted to a single bond and two
new bonds are formed to the species it
reacts with is known as an addition reaction
and they are typical of alkenes and alkynes.
• A number of important addition reactions are
illustrated in the next slides named as:
• Halogenation
• Catalytic Hydrogenation
• Halogen acid addition
• Addition of water
ADDITION OF BROMINE TEST FOR UNSATURATION
The addition of bromine dissolved in tetrachloromethane (CCl4) or
water (known as bromine water) is used as a test for unsaturation. If
the reddish-brown colour is removed from the bromine solution, the
substance possesses a C=C bond or unsaturation.
A
PLACE A SOLUTION OF BROMINE
IN A TEST TUBE
B
ADD THE HYDROCARBON TO BE
TESTED AND SHAKE
C
IF THE BROWN COLOUR
DISAPPEARS THEN THE
HYDROCARBON IS AN ALKENE
A
B
C
Because the bromine adds to the alkene, it no longer
exists as molecular bromine and the typical red-brown
colour disappears
BROMINE WATER TEST MOVIE
B) WITH HYDROGEN – HYDROGENATION
This addition of hydrogen across a double bond happens
only in the presence of a catalyst – usually platinum is
used in the lab reaction. This process converts an alkene
into an alkane.
C) WITH HYDROGEN HALIDES
Note the possibility of isomeric products in this case.
In this reaction we end up with a substituted alkane – a haloalkane.
D) WITH WATER – HYDRATION
In all of the above addition reactions, an unsaturated compound
becomes fully saturated.
ADDITION REACTIONS OF ALKYNES
As the alkynes are unsaturated we might expect that they will
undergo addition reactions like the alkenes. This is indeed
the case but the reaction can happen in two stages and,
with care, can be stopped after the first stage.
a) WITH HALOGENS – TO MAKE DIHALOALKENES, THEN
TETRAHALOALKANES
B) WITH HYDROGEN – TO MAKE ALKENES AND THEN
ALKANES
C) WITH HYDROGEN HALIDES – TO MAKE HALOALKENES
AND THEN DIHALOALKANES
ELECTROPHILIC ADDITION MECHANISM
The electrophile, having
some positive character, is
attracted to the alkene.
The electrons in the pi bond
come out to form a bond to
the positive end.
Because hydrogen can only
have two electrons in its
orbital, its other bond
breaks heterolytically. The
H attaches to one of the
carbon atoms.
ELECTROPHILIC ADDITION MECHANISM
A carbocation is formed. The species that left now has a
lone pair.
It acts as nucleophile and attacks the carbocation using
its lone pair to form a covalent bond. Overall, there is
ADDITION
ELECTROPHILIC ADDITION OF HYDROGEN BROMIDE
Reagent
Condition
Equation
Hydrogen bromide... it is electrophilic as the H is slightly positive
Room temperature.
C2H4(g) + HBr(g) ———> C2H5Br(l)
bromoethane
Mechanism
Step 1
As the HBr nears the alkene, one of the carbon-carbon bonds breaks
The pair of electrons attaches to the slightly positive H end of H-Br.
The HBr bond breaks to form a bromide ion.
A carbocation (positively charged carbon species) is formed.
Step 2
The bromide ion behaves as a nucleophile and attacks the carbocation.
Overall there has been addition of HBr across the double bond.
ADDITION TO UNSYMMETRICAL ALKENES
ELECTROPHILIC ADDITION TO PROPENE
Problem
• addition of HBr to propene gives two isomeric brominated compounds
• HBr is unsymmetrical and can add in two ways
• products are not formed to the same extent
• the problem doesn't arise in ethene because it is symmetrical.
Mechanism
Two possibilities
ADDITION TO UNSYMMETRICAL ALKENES
MARKOWNIKOFF’S RULE
A Russian scientist, Markownikoff, investigated the
products of the addition of hydrogen halides to alkenes.
He found that, when two products were formed, one was
formed in a larger quantity. His original rule was based
only on this reaction. The modern version uses
carbocation stability as a criterion for predicting the
products.
In the electrophilic addition to alkenes the major
product is formed via the more stable carbocation
(carbonium ion)
Carbocation Stability
Build up of charge in one place leads to instability. If it
can be spread around or neutralised in some way,
stability is increased. Alkyl groups are electron releasing
and can “push” electrons towards the carbocations thus
reducing the charge density.
least stable
most stable
methyl <
primary (1°) < secondary (2°) < tertiary (3°)
ADDITION TO UNSYMMETRICAL ALKENES
MARKOWNIKOFF’S RULE
In the addition to propene, path A involves a 2° carbocation, path B a 1° carbocation.
As the 2° ion is more stable, the major product (i.e. 2-bromopropane) is formed this way.
PATH A
SECONDARY
CARBOCATION
MAJOR PRODUCT
PATH B
PRIMARY
CARBOCATION
MINOR PRODUCT
ADDITION TO UNSYMMETRICAL ALKENES
MARKOWNIKOFF’S RULE
When an unsymmetrical reagent adds to the
double bond:
• the positive part (electrophile) of the reagent
will join to the to the carbon atom containing
more number of hydrogen atoms
• the negative part (nucleophile) of the reagent
will join to the carbon atom containing less
number of hydrogen atoms
MECHANISM FOR REACTION OF ALKYNES WITH HBr
Step 1:
Protonation of the alkyne developing +ve charge
on the more substituted carbon.
Step 2:
The other part is attack of the nucleophilic bromide
ion on the more electrophilic carbocation creates
the alkenyl bromide.
Step 3:
In the presence of excess reagent, a second
protonation occurs to generate the more stable
carbocation.
Step 4:
Attack of the nucleophilic bromide ion on the
electrophilic carbocation creates the geminal
dibromide.
OTHER ADDITION REACTIONS
DIRECT HYDRATION
Reagent
steam
Conditions high pressure
Catalyst
(H3PO4)
Product
sulphuric acid ( H2SO4)or phosphoric acid
alcohol
Equation
C2H4(g) +
H2O(g)
Use
ethanol manufacture
C2H5OH(g)
Comments It may be surprising that water needs such
vigorous conditions to react with ethene. It is a highly polar
molecule and you would expect it to be a good electrophile.
However, the O-H bonds are very strong so require a great
deal of energy to be broken. This necessitates the need for a
catalyst.
OTHER ADDITION REACTIONS
HYDROGENATION
Reagent
hydrogen
Conditions nickel catalyst - finely divided
Product
Equation
alkanes
C2H4(g) +
H2(g)
———>
C2H6(g)
ethane
Use
margarine manufacture
ELECTROPHILIC ADDITION OF BROMINE
Reagent
Condition
Equation
Bromine. (Neat liquid or dissolved in tetrachloromethane, CCl4 )
Room temperature. No catalyst or UV light required!
C2H4(g) + Br2(l) ——> CH2BrCH2Br(l)
1,2 - dibromoethane
Mechanism
It is surprising that bromine
should act as an electrophile
as it is non-polar.
SEE NEXT SLIDE FOR AN EXPLANATION OF THE BEHAVIOUR OF BROMINE
ELECTROPHILIC ADDITION OF BROMINE
It is surprising that bromine should act as an electrophile as it is non-polar.
Explanation ...
as a bromine molecule approaches an alkene, electrons in
the pi bond of the alkene repel the electron pair in the
bromine-bromine bond thus inducing a dipole.
NON-POLAR
AS A NON-POLAR BROMINE MOLECULE
APPROACHES AN ALKENE, ELECTRONS
IN THE PI ORBITAL OF THE ALKENE
REPEL THE SHARED PAIR OF
ELECTRONS IN THE Br-Br BOND
POLAR
THE ELECTRON PAIR IS NOW NEARER
ONE END SO THE BROMINE MOLECULE IS
POLAR AND BECOMES ELECTROPHILIC.
Reduction to Alcohols with Sodium Borohydride
and Lithium Aluminum Hydride
CARBONYL COMPOUNDS - REDUCTION WITH NaBH4
Reagent
Conditions
Mechanism
Nucleophile
Product(s)
Equation(s)
Notes
sodium tetrahydridoborate(III) (sodium borohydride), NaBH4
aqueous or alcoholic solution
Nucleophilic addition (also reduction as it is addition of H¯)
H¯ (hydride ion)
Alcohols Aldehydes are REDUCED to primary (1°) alcohols.
Ketones are REDUCED to secondary (2°) alcohols.
CH3CHO + 2[H]
——> CH3CH2OH
CH3COCH3 + 2[H] ——> CH3CHOHCH3
The water provides a proton
CARBONYL COMPOUNDS - REDUCTION WITH NaBH4
Reagent
sodium tetrahydridoborate(III) (sodium borohydride), NaBH4
Conditions
aqueous or alcoholic solution
Mechanism
Nucleophilic addition (also reduction as it is addition of H¯)
Nucleophile
H¯ (hydride ion)
Aldehyde
Primary alcohol
Water is added
CARBONYL COMPOUNDS - REDUCTION WITH NaBH4
Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4
Conditions
aqueous or alcoholic solution
Mechanism
Nucleophilic addition (also reduction as it is addition
of H¯)
Nucleophile
H¯ (hydride ion)
ANIMATED MECHANISM
Grignard Addition - Preparation of Alcohols
• Grignard reagents are prepared from the
reaction of alkyl halides with magnesium in
ether solvent.
• The alkyl group assumes a negative
character and is a nucleophile.
• When presented with an aldehyde or ketone,
the Grignard attacks the carbonyl carbon in a
base-initiated nucleophilic addition.
• Neutralization of the negative intermediate
results in the preparation of an alcohol.
• Grignard reagents react with formaldehyde
to form primary alcohols, with other
aldehydes to form secondary alcohols, and
with ketones to produce tertiary alcohols.
Grignard Preparation of Alcohols
Grignard Reaction Mechanism
Polymers
• Polymers are long chain molecules that are formed by
the joining together of a large number of repeating
units, called monomers, by a process of polymerisation.
• Polymers,can be made artificially and these are usually
referred to as plastics, but there are also a great number
of naturally occurring polymers.
• One type of polymerisation reaction is known as
addition polymerisation. In this the monomers contain
double bonds and in the addition reaction new bonds
(shown coloured below) form between these monomer
units.
• The simplest polymerisation reaction of this type is that
of ethene when heated under pressure with a catalyst to
form polyethene, commonly known as ‘polythylene’.
Polythylene
• polyethylene formation may also be
represented by the equation below in which
the repeating unit is shown in square
brackets.
Polyvinyl Chloride (PVC)
Another common
addition polymer is
poly(chloroethene),
better known as PVC
(short for its old
name of PolyVinyl
Chloride), formed by
the polymerisation
of chloroethene
Monomer
Chloroethene
Polymer
Polyvinyl chloride (PVC)
Monomer
Tetrafluoroethene
Polymer
Polytetrafluoroethene (Teflon)
Polypropene
Polypropene
is another
common
adition
polymers.
REFERENCES
• http://chemweb.calpoly.edu/cbailey/Bailey
Text/SGpdf/Chapter11.pdf
• http://www.knockhardy.org.uk/ppoints.htm
• http://www.chem.ucalgary.ca/courses/350/
Carey5th/Ch09/ch9-8.html