17. Types of Reactions in Organic Chemistry

addition reaction

a chemical reaction in which two or more molecules react together to form a single molecule

addition reaction examples

ethene + hydrogen -> ethane, ethene + bromine -> 1,2-dibromoethane, ethene + chlorine -> 1,2-dichloroethane, ethene + hydrogen chlorice -> chloroethane, ethene + water -> ethanol

addition reactions occur with…

alkenes

addition reaction change in geometry

planar -> tetrahedral

addition reaction change in saturation

unsaturated -> saturated

ethene + hydrogen -> ethane equation

C2H4 + H2 --[Ni]--> C2H6

what reagent and metal cataylst are used to synthesise ethane from ethene?

H2 and Ni

does benzene undergo addition reactions? why or why not?

benzene does not undergo addition reactions due to the stability of its delocalised electrons

mechanism of a reaction

the detailed step-by-step description of how the overall reaction occurs

ionic addition mechanism steps

polarisation, heterolytic fission, carbonium ion formation, ionic addition

ionic addition: ethene and bromine - polarisation

Br2 molecule becomes polarised

ionic addition: ethene and bromine - heterolytic fission

two electrons end up on one atom to form two charged atoms

ionic addition: ethene and bromine - carbonium ion formation

Br+ ion attacks C=C bond for an electron and bonds with one C atom, the other C atom becomes + charged

ionic addition: ethene and bromine - ionic addition

carbonium ion attacked by Br- ion to form 1,2-dibromoethane

what is 1,2-dibromoethane used for?

controlling moths in beehives

what is the supporting evidence for the ionic addition mechanism?

the products 1,2-dibromoethane, 1-bromo,2-chloroethane, and 2-bromoethanol when ethene reacts with water that contains sodium chloride proves that the intermediate of the reaction is a carbonium ion

ionic addition: ethene and chlorine - polarisation

Cl2 molecule becomes polarised

ionic addition: ethene and chlorine - heterolytic fission

two electrons end up on one atom to form two charged atoms

ionic addition: ethene and chlorine - carbonium ion formation

Cl+ ion attacks C=C bond for an electron and bonds with one C atom, the other C atom becomes + charged

ionic addition: ethene and chlorine - ionic addition

carbonium ion attacked by Cl- to form 1,2-dichloroethane

ionic addition: ethene and hydrogen chloride - polarisation

polar HCl molecule becomes even more polarised

ionic addition: ethene and hydrogen chloride

two electrons end up on one atom and two charged atoms are formed

ionic addition: ethene and hydrogen chloride - carbonium ion formation

H+ ion attacks C=C bond for an electron and bonds with one C atom, the other C atom becomes + charged

ionic addition: ethene and hydrogen chloride - ionic addition

carbonium ion attacked by Cl- ion to form chloroethane

polymer

a large molecule made up on many identical repeating units called monomers

polymerisation is a type of…

addition reaction

what reaction is this and what does it form?

polymerisation of ethene to form polythene

what is the monomer and polymer in this reaction?

monomer - ethene, polymer - polythene

polyethene uses

plastic bags, lunchboxes

what reaction is this and what does it form?

polymerisation of propene to form polypropene

what is the monomer and polymer in this reaction?

monomer - propene, polymer - polypropene

polypropene uses

plastic jugs, luggage

examples of synthetic products of the petrochemical industry

cosmetics, plastics, detergents, solvents, paints

when drawing the repeating unit of a polymer…

do not draw end atoms, but leave the end bonds empty with dots to represent that it stretches on

elimination reaction

a chemical reaction in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecule

dehydration reaction

elimination reaction in which water is removed

elimination reaction example

dehydrating ethanol to produce ethene

substitution reaction

a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms

substitution reaction example

halogenation of alkanes in the presence of UV light, methane + chlorine → chloromethane + hydrogen chloride

free radical

any atom or group of atoms with an unpaired electron

monochlorination of methane - initiation

Cl2 molecule broken down into two Cl free radicals in the presence of UV (homolytic fission)

monochlorination of methane - propagation

a Cl atom attacks a methane molecule to form hydrogen chloride and a methyl free radical

monochlorination of methane - propagation (part 2 electric boogaloo)

a methyl free radical attacks a Cl molecule to form chloromethane and a Cl atom, a chain reaction occurs

chain reaction

a reaction that continues on and on because a product from one step of the reaction is a reactant for another step of the reaction

monochlorination of methane - termination

Cl2, chloromethane, and ethane are formed to stop the chain reaction

what is the supporting evidence for the free radical substitution reaction?

the reaction will happen even if UV light is only used for a very short period since it causes a chain reaction, traces quantities of ethane/butane are found which could only be formed from two free radicals, these reactions are sped up by the addition of a known souce of free radicals (tetramethyl lead for methane, tetraethyl lead for ethane)

chlorination of ethane - initiation

a Cl2 molecule is broken down into two Cl free radicals in the presence of UV light

chlorination of ethane - propagation

a Cl atom attacks an ethane molecule to form hydrogen chloride and an ethyl free radical

chlorination of ethane - propagation (part 2 electric boogaloo)

an ethyl free radical attacks a Cl molecule to form chloroethane and a Cl atom, a chain reaction occurs

chlorination of ethane - termination

Cl2, chloroethane, and butane are formed to stop the chain reaction

fully halogenated alkanes are used in…

fire extinguishers

why are fully halogenated alkanes often used in fire extinguishers?

they are not very combustible

why is esterification a substitution reaction?

the OH of the carboxylic acid is replaced by the O and alkyl group of the alcohol

what is the reverse of esterification?

hydrolysis

hydrolysis

the chemical decomposition of a substance by water

base hydrolysis of esters is called

saponification

redox reaction

a chemical reaction in which there is transfer of electrons from one chemical species to another

primary alcohols are oxidised to…

aldehydes, then to carboxylic acids

what reagent is used to synthesise an aldehyde/carboxylic acid from a primary alcohol?

acidified KMnO4

secondary alcohols are oxidised to…

ketones

ketones are…

difficult to oxidise

aldehydes and ketones can be reduced to…

alcohols

name the reagent and transition metal catalyst used to reduce aldehydes/ketones to alcohols

hydrogen and nickel (H2/Ni)

carboxylic acids can be reduced back to…

aldehydes and alcohols

what reagent and metal catalyst is used to reduce carboxylic acids back to aldehydes and alcohols?

hydrogen and nickel catalyst (H2/Ni)

write a balanced equation that shows that alcohol can act as an acid

C2H2OH + Na —> C2H5O-Na+ + ½H2

why can alcohols act as an acid?

the polarity of the OH bond makes it easier for the H atom to break off as an H+ ion

why can carboxylic acids act as an acid?

the inductive effect and the stability of the carboxylate ion

draw a diagram to show the acidic nature of carboxylic acids and explain

the two carbon-oxygen bonds are equal length. the carboxylate ion is so stable it makes the carboxylic acid want to lose its proton

organic synthesis

the process of making organic compounds from simpler starting materials

useful products of organic synthesis

aspirin, penicillin, PVC

synthesis of PVC from ethene step 1

ethene reacts with chlorine to form 1,2-dichloroethane (first intermediate)

synthesis of PVC from ethene

1,2-dichloroethane is subjected to thermal cracking which removes HCl to form chloroethene (second intermediate)

synthesis of PvC from ethene

chloroethene is polymerised to poly(chloroethene) which is also called polyvinyl chloride (PVC)

what is the principle of the separation of the components in a mixture using any type of chromatography?

the principle on which all chromatographic separation techniques are based is that separation of a mixture of components occurs as a result of selective adsorbance of the components of the mixture on a stationary phase while carried by a mobile phase

paper chromatography phases

mobile - water, stationary - paper

gas chromatography phases

mobile - gas, stationary - non-volatile liquid spread on an inert solid

gas chromatography processes

injection, transport of the sample along the column, separation in the column, detection

high performance liquid chromatography (HPLC) phases

mobile - solvent, stationary - very fine particles of silica

HPLC processes

injection, transport of the sample along the column, separation in the column, detection

gas chromatography uses

measure levels of alcohol in blood

HPLC uses

measure levels of alcohol in blood

infrared spectrometry principal

the principal of infrared spectrometry is that organic compounds absorb IR radiation of certain frequencies

what does infrared spectrometry do?

identifies organic compounds

infrared spectrometry processes

IR radiation is passed through the sample, IR radiation of a certain fixed frequencies is absorbed, an absorption spectrum is obtain and can be used to identify the compound

ultraviolet spectrometry principle

the principle of ultraviolet absorption spectrometry is that absorbance is directly proportional to concentration

what does ultraviolet spectrometry do?

identifies concentration of an organic compound

ultraviolet spectrometry processes

a solution of the substance being analysed is placed between a source of UV light and a detector, the detector measures the intensity of light reaching it, a UV absorption spectrum is obtained and can be used to identify the concentration of the compound