Presentation on theme: "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."— Presentation transcript:
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 (CCl 4 ) 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 ABCABC 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
ReagentHydrogen bromide... it is electrophilic as the H is slightly positive ConditionRoom temperature. EquationC 2 H 4 (g) + HBr(g) ———> C 2 H 5 Br(l) bromoethane Mechanism Step 1As 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 2The 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 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 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 RULE ADDITION TO UNSYMMETRICAL ALKENES PATH A PATH B MAJOR PRODUCT PRIMARY CARBOCATION SECONDARY 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
DIRECT HYDRATION Reagentsteam Conditionshigh pressure Catalystsulphuric acid ( H 2 SO 4 )or phosphoric acid (H 3 PO 4 ) Productalcohol EquationC 2 H 4 (g) + H 2 O(g) C 2 H 5 OH(g) Useethanol 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 Reagenthydrogen Conditionsnickel catalyst - finely divided Productalkanes EquationC 2 H 4 (g) + H 2 (g) ———> C 2 H 6 (g) ethane Usemargarine manufacture OTHER ADDITION REACTIONS
ReagentBromine. (Neat liquid or dissolved in tetrachloromethane, CCl 4 ) ConditionRoom temperature. No catalyst or UV light required! EquationC 2 H 4 (g) + Br 2 (l) ——> CH 2 BrCH 2 Br(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. 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-POLARPOLAR
Reduction to Alcohols with Sodium Borohydride and Lithium Aluminum Hydride
Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH 4 Conditionsaqueous or alcoholic solution MechanismNucleophilic addition (also reduction as it is addition of H¯) NucleophileH¯ (hydride ion) Product(s)Alcohols Aldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols. Equation(s)CH 3 CHO + 2[H] ——> CH 3 CH 2 OH CH 3 COCH 3 + 2[H] ——> CH 3 CHOHCH 3 NotesThe water provides a proton CARBONYL COMPOUNDS - REDUCTION WITH NaBH 4
Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH 4 Conditionsaqueous or alcoholic solution MechanismNucleophilic addition (also reduction as it is addition of H¯) NucleophileH¯ (hydride ion) CARBONYL COMPOUNDS - REDUCTION WITH NaBH 4 Water is added Primary alcohol Aldehyde
Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH 4 Conditionsaqueous or alcoholic solution MechanismNucleophilic addition (also reduction as it is addition of H¯) NucleophileH¯ (hydride ion) CARBONYL COMPOUNDS - REDUCTION WITH NaBH 4 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