Chapter 23 - Types of Reactions in Organic Chemistry

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

mechanism of a reaction

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

show movement of electrons represented with arrows

• one electron → half arrow head

• electron pair → full arrow head

example of substitution reaction

alkanes with halogens in presence of UV light

free radicals 

any atom or group of atoms with an unpaired electron 

homolytic fission

process in which a bond breaks in such a way that each atom joined retains one of the two electrons

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

4 steps of the mechanism of a substitution reaction

• initiation 

• propagation 1 

• propagation 2 

• termination 

evidence for mechanism of substitution reaction

• use of ultraviolet light even for very short period of time causes chain reaction

• for every light photon absorbed, thousands of chloromethane atoms formed

• evidence for the combining free radicals comes from fact small amounts of ethane found among products

• adding tetramethyl to reaction increases speed of reaction as it promotes chain reaction by providing free radicals as it decomposes

use of halogenated alkanes

flame retardants

→ halogenated alkanes don’t support combustion and added to fabrics to reduce tendency to catch fire

what are the types of substitution reactions?

• monochlorination

• esterification

• saponification (base-catalysed hydrolysis of esters)

what types of reactions is esterification?

• condensation reaction (water formed)

• substitution reaction (H attached to -OH replaced by alkyl group)

saponification 

esters are hydrolysed in presence of a base

molecular formula of glycerol (propane- 1,2,3-triol)

C₃H₅(OH)₃

addition reaction

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

hydrogenation

addition of hydrogen to a molecule

halogenation reaction

addition of halogens to a molecule

steps for addition reaction between bromine and ethene

1. polarisation

2. heterolytic fission

3. carbonium ion formation

4. ionic addition

uses of 1,2-dibromoethane

• fumigant for treating timber against termites

• control moths in beehives

uses of 1,2-dichloroethane

• used to manufacture chloroethane and make PVC

uses of chloroethane 

• used in dentistry to diagnose a ‘dead tooth’

what differs about reaction between ethene - hydrogen to ethene - bromine

already polarised hydrogen chloride further polarised by repulsion of double bond’s electrons

how is margarine made?

hydrogenation of vegetable oils

polymers

long chain molecules made by joining small molecules, they consist of a repeating pattern 

common polymers

• polythene

• polypropene (plastic beakers)

• PVC (poly(chloroethene))

what are polymers commonly referred to as?

plastics

elimination reaction 

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

dehydration reaction

water is removed during a reaction

(type of elimination reaction)

oxidation

the loss of electrons from a substance

reduction

the gain of electron to a substance

mnemonic for redox reactions

• Oxidation

• Is

• Loss

• Reduction

• Is

• Gain

oxidising agent

substance that is responsible for oxidation of another substance

reducing agent 

substance that is responsible for reduction of another substance