Free radical substitution and nucleophilic substitution

Question 1

define nucleophile

Answer 1

an e- pair donor

contains an area of high e- density (typically negatively charged)

Question 2

define free radical

Answer 2

an atom or group of atoms which have a single unpaired electron (shown as a single black dot next to the atom to which it belongs)

occurs when due to homolytic fission (when a bond pair of e- is split evenly and each of the atoms gets an e-)

Question 3

what is the main use of free radical substitution?

Answer 3

to make haloalkanes-one or more of the H atoms are substituted by a halogen atom

Question 4

name and outline the mechanism for this reaction including each step

CH₄ + Cl₂ → CH₃Cl + HCl

Answer 4

this is an example of free radical substitution

First step: initiation

Cl₂ → 2Cl• (in the presence of UV light)

Step 2: propagation

the chlorine free radical steals a H atom from the alkane

a dot is placed above the C which has lost a H atom

Cl• + CH4 → •CH3+ HCl

the newly formed free radical then reacts with Cl₂ to form the haloalkane ( a Cl atom is stolen )

•CH₃ + Cl₂ → CH₃Cl + Cl•

This reforms the chlorine free radicals which can react with the other methane molecules and so on, resulting in a chain reaction

Step 3: Termination

when the free radicals combine, the reaction terminates

2Cl• → Cl₂

2•CH₃ → CH₃CH₃

Cl• + •CH₃ → CH₃Cl (chloromethane also formed here)

Question 5

Write the free radical substitution equations which occur in the ozone layer and describe why this process is a problem

Answer 5

Cl• + O₃ ⟶ ClO• + O_{<span>2</span>}

ClO• + O₃ ⟶ 2O₂ + Cl•

Ozone (O₃) is an allotrope of oxygen found in the stratosphere where it absorbs some of the harmful UV radiation from the sun. UV light can damage plant tissue, reduce plankton populations in the ocean and cause skin cancer

Ozone levels have been decreasing due to the free radical substitution reactions with halogen free radicals which result in its break down to O₂

Cl free radicals are the most dangerous. They are found in CFCs which are used as fire extinguishers and refrigerants. When CFCs enter the stratosphere, the UV light results in the homolysis of the C-Cl bond releasing the Cl•

Question 6

why do unsaturated compound not react with nucleophile

Answer 6

the -C=C- bond is an are of high e- density so repels the nucleophile (as it has a negative charge)

Question 7

draw a diagram of reflux apparatus

Answer 7

Question 8

draw a diagram of the apparatus needed for simple distillation

Answer 8

Question 9

Outline the mechanism and state the type of reaction and the conditions needed for the reaction between a haloalkane and KOH

Answer 9

conditions:

• Dissolve haloalkane in a small volume of ethanol
• Add aqueous solution of NaOH (or any other hydroxide ion containing compound)
• Warm reflux

type of reaction = hydrolysis

Question 10

Outline the mechanism and state the conditions needed for the reaction between a haloalkane and KCN

Answer 10

conditions

• Dissolve haloalkane in a small volume of ethanol
• Add aqueous solution of KCN (or any other cyanide ion containing compound)
• Warm reflux

Question 11

Outline the mechanism and state the conditions needed for the reaction between a haloalkane and NH₃

Answer 11

conditions:

• Dissolve haloalkane in a small volume of ethanol
• Add excess concentrated ammonia solution
• Seal container under high pressure

Question 12

How would you increase the likelihood that a primary amine is formed from the nucleophilic substitution of NH₃ into a haloalkane

Answer 12

• to prevent primary and secondary amines from act as nucleophiles to react with the the haloalkane and result in further substitution, add excess NH₃
• this means it is more likely for a :NH₃ nucleophile to react with the haloalkane

Question 13

define reflux and explain why its useful

Answer 13

• continuous boiling and condensing of a reaction mixture
• helps increase the rate of reaction and yield of products as nucleophilic reactions are too slow at room temp

Question 14

expalin why the flask in reflux apparatus is not sealed and why anti-bumping granules are added

Answer 14

• not sealing the flask prevents the build up of pressure inside the apparatus
• anti-bumping granules ensure smooth boiling

Question 15

explain why haloalkanes are able to partake in nucleophilic substitution reactions

Answer 15

• C delta + of polar C-X bond is susceptible to nucleophilic attack

Question 16

explain how you would compare the reactivity of different haloalkanes using hydrolysis reactions

Answer 16

• Add a known amount of haloalkane and the same amount of AgNO3 solution in a mixture of ethanol and water. Keep the temp the same
• Measure the time taken for a specific amount of precipitate to form (or draw a black X on test tube and measure time taken for X to disappear)
• Calculate rate using amount of ppt/time or 1/time
• The ethanol and water form hydroxide ion nucleophiles which go on to react with the haloalkane to form an alcohol and halide ion leaving groups
• The precipitate forms when the halide ion has been released from the haloalkane
• The weaker the C-X bond in the haloalkane, the easier it is to release the halide ions, so the faster the formation of the precipitate=higher RoR=higher reactivity

Question 17

what is the general formula for haloalkanes?

Answer 17

C_nH_2n+1X

Question 18

outline the mechanism and state the conditions for the elimination reaction of a haloalkane

Answer 18

conditions:

• KOH (or any other OH containing compound) is dissolved in ethanol
• Anhydrous
• Hot reflux

what happens:

role of :OH nucleophile = act as a base (H+ acceptor)

Step 1) A H+ is removed from the C atom next to the C-X bond

Step 2) A C=C bond forms between the C next to the C-X and the C in the C-X bond

Step 3) A H₂O molecule and a halide ion leaving group are formed

Question 19

state which mechanism is more likely to occur between a :OH^- nucleophile and a 1º, 2º or 3º haloalkane

Answer 19

1º haloalkane = substitution more likely

2º haloalkane = equally likely for either substitution or elimination

3º = elimination more likely

Question 20

define electrophile

Answer 20

• electron pair acceptor
• They are electron deficient species so are attracted by areas of high electron density
• Often polar molecules

Question 21

why do alkenes undergo electrophilic addition?

Answer 21

the C=C bond is an area of high e- density so is susceptible to electrophilic attack

Question 22

define heterolytic fission and give two examples where it happens

Answer 22

• when the covalent bond is broken unevenly so one atom receives both e- while the other receives none
• this produces a positive ion and a negative ion
• nucleophilic substitution and electrophilic addition involve heterolytic fissions

Question 23

Outline the mechanism between an alkene and HBr

Answer 23

Question 24

outline the mechanism between an alkene and Br₂

Answer 24

When the neutral molecule approaches the C=C bond, a temporary dipole is induced because the high e- density repels the electrons in the neutral molecule more towards one of the Br atoms

no major/minor products as product is always the same no matter which carbocation used