Reactions in Organic Chemistry - Substitution Reaction

Question 1

Substitution reaction

Answer 1

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

Question 2

example of substitution reaction

Answer 2

Alkanes reaction w/ halogens in presence of ultraviolet light

Question 3

Monochlorination

Answer 3

replacing just one atom of hydrogen by one atom of chlorine (called monochlorination of methane)

Question 4

HCl

Answer 4

not hydrochloric acid in this, it is hydrogen chloride

Question 5

Mechanism

Answer 5

mechanism of a reaction is the detailed step-by-step description of how the overall reaction occurs

Question 6

mechanism of reaction use

Answer 6

needed in order to establish the best conditions of a particular reaction ( to maximise yield of products)

Question 7

chemical reactions

Answer 7

involve bond breaking + bond formation.

When bonds broken/made, there is always a rearrangement of electrons

Question 8

curly arrows

Answer 8

half of head of arrow - movement of 1 electron

full of head of arrow - movement of electron pair

Question 9

mechanism of monochlorination of methane - steps

Answer 9

1. initiation
2. propagation
3. propagation
4. termination

Question 10

mechanism of monochlorination of methane -

Step 1: initiation

Answer 10

1. Chlorine molecule broken down into two chlorine atoms in prsence of uv light. Uv light provides energy to break bond in chlorine molecule + convert chlorine molecule into two chlorine free readicals
2. Retains one of 2 electrons from bond. This type of bond breaking is called homolysis/homolytic fission
3. Key feature is that each of two products ends up w/ one of pair of electrons involved in bonding togethe
4. This reaction is example of photochemical reaction.
   - evidence of photochemical reaction

Question 11

mechanism of monochlorination of methane -

Evidence for photochemical reaction

Answer 11

Evidence comes from fact reaction doesnt occur in dark at room temp
(photochemical reaction)

Question 12

mechanism of monochlorination of methane -

Step 2: propagation

Answer 12

1. Chlorine atom attacks methane molecule to form hydrogen chloride + species called methly free radical (any atom/group of atoms w unpaired electron. Symbol is as dot)
2. Propagation involves use of free radical to produce another free radical. Since they do not have full octet of electrons in outer energy level, are v. reactive + usually only exist for v. short periods.

Question 13

mechanism of monochlorination of methane -

Step 3: propagation

Answer 13

1. Methyl free radical attacks chlorine molecule to form chloromethane + a chlorine atom
2. Chlorine atom can react w/ another molecule of methane. Sets up chain reaction (continues on bc a product from one step is reactant for another step)
   - chain reaction evidence
3. Found that for every photon absorbed, many thousands of molecules of chloromethane were produced

Question 14

mechanism of monochlorination of methane -

Chain reaction evidence

Answer 14

Evidence for this chain reaction comes from a study of the amount of light absorbed by the glass container in which reaction is taking place.

Found that for every photon absorbed, many thousands of molecules of chloromethane were produced.

Question 15

mechanism of monochlorination of methane -

Step 4: termination

Answer 15

1. Number of reactions take place to stop chain reaction. Involve free radicals.
2. Reaction comes to an end in this stage.
3. Chain reaction can go thru hundreds/thousands cycles before termination. Reactions extremely fast + reaction has explosive nature
   - evidence of free radicals combining
4. Tetramethyl lead inc. rate of reaction
5. Chlorination of methane is commonly referred to as a free radical substitution reaction

Question 16

mechanism of monochlorination of methane -

Combining of free radicals evidence

Answer 16

Evidence for combining of free radicals comes from fact that small amounts of ethane are found among the products. Ethane must have come from the combination of two methyl free radicals.

Question 17

Mechanism for the chlorination of ethane

Answer 17

Same mech holds for chlorination of ethane as chlorination of methane

Radical formed is ethyl radical

termination stage - compound formed will be butane

Question 18

Uses of fully halogenated alkanes

Answer 18

Fully halogenated alkanes are added to fabrics as flame retardants

Question 19

Esterification - what its also called

Answer 19

Called a condensation reaction since water formed in reaction. Also substitution since H attached to -OH group is carboxylic acid molecule has been replaced by an alkyl group

Question 20

Esterification

Answer 20

Formation of an ester, an equilibrium reaction

Question 21

reverse esterification

Answer 21

hydrolysis

Esters are hydrolysed v. effectively in presence of a base

Question 22

Hydrolysis of esters

Answer 22

• forms the sodium salt of the carboxylic acid rather than the carboxylic acid itself
• used in manufacture of soap (saponification)
• Catalytic step involves the OH⁻ ions from NaOH + results in production of acid + alcohol
• Acid then donates H⁺ to the OH⁻ ions forming water + carboxylate ions