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Transcript
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. 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 CH3 + CH3 ~ 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acid react together, (condensation reaction) ~ The H attached to the -OH group in the carboxylic acid molecule has been replaced by the alkyl group, it is a type of substitution reaction.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 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 - bromo2 - 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 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 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