2 REACTIONS OF ALKENESA 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:HalogenationCatalytic HydrogenationHalogen acid additionAddition of water
4 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.ABCPLACE A SOLUTION OF BROMINE IN A TEST TUBEADD THE HYDROCARBON TO BE TESTED AND SHAKEIF THE BROWN COLOUR DISAPPEARS THEN THE HYDROCARBON IS AN ALKENEA B CBecause the bromine adds to the alkene, it no longer exists as molecular bromine and the typical red-brown colour disappears
6 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 HALIDESNote the possibility of isomeric products in this case.In this reaction we end up with a substituted alkane – a haloalkane.
7 D) WITH WATER – HYDRATION In all of the above addition reactions, an unsaturated compound becomes fully saturated.
8 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.WITH HALOGENS – TO MAKE DIHALOALKENES, THEN TETRAHALOALKANES
9 B) WITH HYDROGEN – TO MAKE ALKENES AND THEN ALKANESC) WITH HYDROGEN HALIDES – TO MAKE HALOALKENES AND THEN DIHALOALKANES
10 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.
11 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
12 ELECTROPHILIC ADDITION OF HYDROGEN BROMIDE Reagent Hydrogen bromide... it is electrophilic as the H is slightly positiveCondition Room temperature.Equation C2H4(g) HBr(g) ———> C2H5Br(l) bromoethaneMechanismStep 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.
13 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.MechanismTwo possibilities
14 ADDITION TO UNSYMMETRICAL ALKENES MARKOWNIKOFF’S RULEA 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)
15 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 stablemethyl < primary (1°) < secondary (2°) < tertiary (3°)
16 ADDITION TO UNSYMMETRICAL ALKENES MARKOWNIKOFF’S RULEIn 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 ASECONDARYCARBOCATIONMAJOR PRODUCTPATH BPRIMARYCARBOCATIONMINOR PRODUCT
17 ADDITION TO UNSYMMETRICAL ALKENES MARKOWNIKOFF’S RULEWhen 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 atomsthe negative part (nucleophile) of the reagent will join to the carbon atom containing less number of hydrogen atoms
19 OTHER ADDITION REACTIONS DIRECT HYDRATIONReagent steamConditions high pressureCatalyst sulphuric acid ( H2SO4)or phosphoric acid (H3PO4)Product alcoholEquation C2H4(g) H2O(g) C2H5OH(g)Use ethanol manufactureComments 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.
22 ELECTROPHILIC ADDITION OF BROMINE 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) ,2 - dibromoethaneMechanismIt is surprising that bromineshould act as an electrophileas it is non-polar.SEE NEXT SLIDE FOR AN EXPLANATION OF THE BEHAVIOUR OF BROMINE
23 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.NON-POLARPOLARAS 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 BONDTHE ELECTRON PAIR IS NOW NEARER ONE END SO THE BROMINE MOLECULE IS POLAR AND BECOMES ELECTROPHILIC.
25 Reduction to Alcohols with Sodium Borohydride and Lithium Aluminum Hydride
26 CARBONYL COMPOUNDS - REDUCTION WITH NaBH4 Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4Conditions 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 [H] ——> CH3CH2OHCH3COCH [H] ——> CH3CHOHCH3Notes The water provides a proton
27 CARBONYL COMPOUNDS - REDUCTION WITH NaBH4 Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4Conditions aqueous or alcoholic solutionMechanism Nucleophilic addition (also reduction as it is addition of H¯)Nucleophile H¯ (hydride ion)AldehydePrimary alcoholWater is added
28 CARBONYL COMPOUNDS - REDUCTION WITH NaBH4 Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4Conditions aqueous or alcoholic solutionMechanism Nucleophilic addition (also reduction as it is addition of H¯)Nucleophile H¯ (hydride ion)ANIMATED MECHANISM
29 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.
32 PolymersPolymers 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’.