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

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.

ADDITION OF BROMINE TEST FOR UNSATURATION

PLACE A SOLUTION OF BROMINE IN A TEST TUBE

ADD THE HYDROCARBON TO BE TESTED AND SHAKE

IF THE BROWN COLOUR DISAPPEARS THEN THE HYDROCARBON IS AN ALKENE

A

B

C

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.

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 MECHANISM

Reagent Hydrogen bromide... it is electrophilic as the H is slightly positive

Condition Room temperature.

Equation C2H4(g) + HBr(g) ———> C2H5Br(l) bromoethane

Mechanism

Step 1 As the HBr nears the alkene, one of the carbon-carbon bonds breaksThe 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.

ELECTROPHILIC ADDITION OF HYDROGEN BROMIDE

ADDITION TO UNSYMMETRICAL ALKENESADDITION TO UNSYMMETRICAL ALKENES

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

ELECTROPHILIC ADDITION TO PROPENE

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)

MARKOWNIKOFF’S RULE

ADDITION TO UNSYMMETRICAL ALKENESADDITION TO UNSYMMETRICAL ALKENES

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°)

Carbocation Stability

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.

MARKOWNIKOFF’S RULEADDITION TO UNSYMMETRICAL ALKENESADDITION TO UNSYMMETRICAL ALKENES

PATH A

PATH B

MAJOR PRODUCT

PRIMARYCARBOCATION

SECONDARYCARBOCATION

MINOR PRODUCT

ADDITION TO UNSYMMETRICAL ALKENESADDITION 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.

DIRECT HYDRATIONReagent steam

Conditions high pressure

Catalyst sulphuric acid ( H2SO4)or phosphoric acid (H3PO4)

Product alcohol

Equation C2H4(g) + H2O(g) C2H5OH(g)

Use ethanol manufacture

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 alkanes

Equation C2H4(g) + H2(g) ———> C2H6(g)

ethane

Use margarine manufacture

OTHER ADDITION REACTIONS

Reagent Bromine. (Neat liquid or dissolved in tetrachloromethane, CCl4 )

Condition Room temperature. No catalyst or UV light required!

Equation C2H4(g) + Br2(l) ——> CH2BrCH2Br(l) 1,2 - dibromoethane

Mechanism

It is surprising that bromineshould act as an electrophileas 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 inthe pi bond of the alkene repel the electron pair in thebromine-bromine bond thus inducing a dipole.

ELECTROPHILIC ADDITION OF BROMINE

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

THE ELECTRON PAIR IS NOW NEARER ONE END SO THE BROMINE MOLECULE IS POLAR AND BECOMES ELECTROPHILIC.

NON-POLAR POLAR

Reduction to Alcohols with Sodium Borohydrideand Lithium Aluminum Hydride

Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4

Conditions aqueous or alcoholic solutionMechanism Nucleophilic addition (also reduction as it is addition of H¯)Nucleophile H¯ (hydride ion)Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols.

Ketones are REDUCED to secondary (2°) alcohols.Equation(s) CH3CHO + 2[H] ——> CH3CH2OH

CH3COCH3 + 2[H] ——> CH3CHOHCH3

Notes The water provides a proton

CARBONYL COMPOUNDS - REDUCTION WITH NaBHCARBONYL COMPOUNDS - REDUCTION WITH NaBH44

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)

CARBONYL COMPOUNDS - REDUCTION WITH NaBHCARBONYL COMPOUNDS - REDUCTION WITH NaBH44

Water is added

Primary alcoholPrimary alcoholAldehydeAldehyde

Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4

Conditions aqueous or alcoholic solutionMechanism Nucleophilic addition (also reduction as it is

addition of H¯)Nucleophile H¯ (hydride ion)

CARBONYL COMPOUNDS - REDUCTION WITH NaBHCARBONYL COMPOUNDS - REDUCTION WITH NaBH44

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 Polymer

Chloroethene Polyvinyl chloride (PVC)

Monomer Polymer

Tetrafluoroethene Polytetrafluoroethene (Teflon)

Polypropene

Polypropene is another common adition polymers.

REFERENCES

• http://chemweb.calpoly.edu/cbailey/BaileyText/SGpdf/Chapter11.pdf

• http://www.knockhardy.org.uk/ppoints.htm

• http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch09/ch9-8.html