TYPES OF REACTIONS IN ORGANIC CHEMISTRY

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Angel Cunningham
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1 TYPES OF REACTIONS IN ORGANIC CHEMISTRY SUBSTITUTION REACTIONS A substitution reaction is a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms. (a) HALOGENATION OF ALKANES Initiation UV light breaks down the Chlorine molecule into two Chlorine atoms (homolytic fission). / Propagation A chlorine atom attacks the methane molecule to form hydrogen chloride and a methyl free radical. A methyl free radical attacks a chlorine molecule to form chloromethane and a chlorine atom. Termination The combining together of free radicals and atoms with each other stops the reaction.

2 EVIDENCE FOR THE MECHANISM ~ the reaction will not occur in the dark at room temperature ~ the energy supplied is not enough to break the C - H bond ~ No molecular hydrogen produced - hence no hydrogen free radicals formed ~ Ethane is produced in small amounts, can only be explained by CH 3 + CH 3 ~ The presence of tetramethyl - lead greatly speeds up the reaction as it is a source of methyl free radicals. (b )ESTERIFICATION ~ Esters are formed when an alcohol and a carboxylic acid react together, (condensation reaction) ~ The H attached to the -OH group in the carboxylic acid molecule has been replaced by the alkyl group, it is a type of substitution reaction. ~ The reverse reaction of esterification is called hydrolysis ADDITION REACTIONS An addition reaction is one in which two substances react together to form a single substance. Polarisation The high concentration of negative charge in the C = C double bond causes the Br - Br bond to be polarised

3 HETROLYTIC FISSION The induced polarisation becomes so great, the Br 2 molecule splits into Br + and Br- (hetrolytic fission) CARBONIUM ION FORMATION The carbonium ion is formed by the Br + attacking the double bond BROMIDE ION ATTACK ON THE CARBONIUM ION EVIDENCE FOR THE MECHANISM );- When bromine water containing sodium chloride reacts with ethane, two other compounds are formed in addition to 1,2 - dibromoethane. These are 1 - bromo- 2 - chloroethane and 2 - dibromoethanol. );- When ethane reacts with bromine in water, 2 - bromoethanol is formed, evidence that the carbonium ion exists );- Chlorine reacts by the same mechanism. );- In the case of HCI, the H+ ion attacks the C = C first. Cl then reacts with the carbonium ion to form chloroethane

4 POL YMERISATION REACTOINS ~ Polymers are long chain molecules made by joining together many small molecules ~ Polymers consist of a repeating structure ~ Monomers are the small molecules from which the polymer is made. ~ Molecules of ethene link with each other to form polythene. ~ Molecules of propene link with each other polypropene ~ Crude oil, a source of alkenes, is the raw material for the manufacture of polymers. ELIMINATION REACTION An elimination reaction is one in which a small molecule is removed from a larger one to leave a double bond in the larger molecule. ~ The removal of water from an alcohol to form an alkene is an elimination reaction (Dehydration reaction) REDOX REACTIONS >- When a primary alcohol reacts with an oxidising agent, it is converted to an aldehyde >- Oxidising agents are sodium dichromate, acidified potassium permanganate >- If the oxidising agent is in excess, the aldehyde is further oxidised to a carboxylic acid. >- Ethanol in alcoholic drinks is oxidised to ethanal in the liver

5 REACTIONS OF ALCOHOLS AND CARBOXYLIC ACIDS AS ACIDS REACTION OF ALCOHOL Sodium reacts with ethanol to form a salt called sodium ethoxide and hydrogen gas ACIDIC NATURE OF THE CARBOXYLIC ACID GROUP ~ The carbonyl group has the effect of drawing electrons away from the - OH group and this facilitates the ionisation of the H atom in the -OH bond. ~ The fact that the carboxylate ion is stabilised by resonance is the driving force for the carboxylic acid to lose a proton. ORGANIC SYNTHESIS Petroleum '\ Alkanes Alkenes ~ ~ Alcohols Alkynes Esters Aldehydes Ketones Carboxylic acid Soaps

Page 1. 6. Which hydrocarbon is a member of the alkane series? (1) 1. Which is the structural formula of methane? (1) (2) (2) (3) (3) (4) (4)

Page 1. 6. Which hydrocarbon is a member of the alkane series? (1) 1. Which is the structural formula of methane? (1) (2) (2) (3) (3) (4) (4) 1. Which is the structural formula of methane? 6. Which hydrocarbon is a member of the alkane series? 7. How many carbon atoms are contained in an ethyl group? 1 3 2 4 2. In the alkane series, each molecule