Chapter 13 - Halogen Derivatives

Question 1

How are halogenoalkanes classified?

Answer 1

they are classified as primary, secondary or tertiary, when there are 1, 2 or 3 alkyl groups attached to the halogenated carbon.

Question 2

What are halogenoarenes?

Answer 2

Also known as aryl halides, these compounds have a halogen atom attached directly to an aromatic carbon ring.

Question 3

What are 3 physical properties of halogenoalkanes?

Answer 3

1) colourless liquids with sweetish smells
2) immiscible with and denser than water
3) boiling points are higher than those of their corresponding alkanes

Question 4

What is the boiling point difference for chloro, bromo and iodoalkane? Why is there such a difference? (2)

Answer 4

choloroalkane < bromoalkane < iodoalkane
1) As the number of electrons of the molecule increases in this order, the size of the electron cloud increases as well. So intermolecular dispersion forces increase in strength in the same order. Energy needed to overcome the strength of intermolecular attractions between molecules
increases in the order of B < C < D, thus boiling point increases in this order.
2) The small amount of pd-pd attractions between molecules is not as significant as dispersion forces, but is more important for chloroalkanes, becoming less so with bromo and iodo since they are less polar.

Question 5

Explain the difference in boiling point between alkanes and halogenoalkanes. (2)

Answer 5

1) Halogenoalkanes has more electrons than alkanes and a larger electron cloud, so halogenoalkanes forms stronger dispersion forces between its molecules than alkanes.
2) there is some amount of permanent dipole-permanent dipole attractions for halogenoalkanes. Thus, more energy is needed to overcome stronger intermolecular attractions between halogenoalkane molecules during boiling.
Therefore halogenoalkanes have a higher boiling point.

Question 6

How can halogenoalkanes be prepared? (3)

Answer 6

1) free radical substitution (alkanes)
2) electrophilic addition of alkenes
3) nucleophilic substitution of alcohols

Question 7

How can halogenoarenes be prepared?

Answer 7

Electrophilic substitution of arenes

Question 8

Why can nucleophilic substitutions occur for halogenoalkanes?

Answer 8

Halogenoalkanes are fairly reactive due to the polarity of the C–X bond (Halogens more reactive than C, making the bond polar). The electron-deficient carbon atom has a partial positive charge which causes it to be attractive to nucleophiles (e donating). The halogen atom will be substituted by the nucleophile.

Question 9

What happens to the stereochemistry of the halogenoalkane after the SN2 mechanism? (2)

Answer 9

1) If the original halogenoalkane molecule is chiral, the stereochemistry at the electron-deficient carbon will be inverted after the substitution reaction.
2) If the reaction occurs purely through SN2, only one of the enantiomers will be formed as a product, and the product will be optically active. (if it is chiral)

Question 10

What happens to the stereochemistry of the halogenoalkane after the SN1 mechanism?

Answer 10

Since the carbocation is trigonal planar with respect to the e-deficient carbon, the nucleophile is able to attack from the top and bottom face of the carbocation with equal probability. If the addition of the nucleophile results in a chiral product, a racemic mixture of both enantiomers will be formed.

Question 11

How does different structures of different classes of halogenoalkanes affect the rate of SN1 mechanism reaction?

Answer 11

The difference in stability affects the rate of reaction.
Alkyl groups donate electrons to the carbocations to help stabilize them. As such, the stability of carbocations increase in the order primary < secondary < tertiary. As the carbocation becomes more stable, the activation energy needed for the reaction decreases.
Thus, the rate of SN1 increases in the order of primary < secondary < tertiary.

Question 12

How does different structures of different classes of halogenoalkanes affect the rate of SN2 mechanism reaction?

Answer 12

The difference in steric hindrance affects the rate of reaction.
The transition state in SN2 has 5 groups arranged around the central carbon atom. The more alkyl groups there are around the central atom, the more crowded the transition state and thus the higher the activation energy, slowing down the SN2 reaction.
Thus, the rate of SN2 increases in the order of tertiary < secondary < primary.

Question 13

Which mechanism will primary halogenoalkanes react by predominantly?

Answer 13

SN2

Question 14

Which mechanism will tertiary halogenoalkanes react by predominantly?

Answer 14

SN1

Question 15

Which mechanism will secondary halogenoalkanes react by predominantly?

Answer 15

mixture of SN1 and SN2

Question 16

How do we compare the reactivity between different halogenoalkanes?

Answer 16

The ease of nucleophilic substitution depends on the ease of breaking the C–X bond. As the size of the halogen atom increases, the C–X bond length increases and the bond strength decreases, and hence nucleophilic substitution takes place more readily.
rate increases in the order F (doesn’t undergo) < Cl < Br < I

Question 17

What is 1 reagent and 1 condition needed for alkaline hydrolysis of halogenoalkanes?

Answer 17

dilute NaOH, heat

Question 18

What is 1 reagent and 1 condition needed for nucleophilic substitution of halogenoalkanes with CN- to form nitriles?

Answer 18

ethanolic KCN, heat

Question 19

What is 1 reagent and 1 condition needed for acidic hydrolysis of nitriles?

Answer 19

dilute H2SO4, heat

Question 20

What is 1 reagent and 1 condition needed for alkalic hydrolysis of nitriles?

Answer 20

dilute NaOH, heat

Question 21

What are the reagents and conditions needed for reduction of nitriles? (2)

Answer 21

1) LiAlH4 in dry ether or

2) Hydrogen gas, Ni catalyst, high pressure

Question 22

What is 1 reagent and 1 condition needed for nucleophilic substitution of halogenoalkanes with NH3 to form amines?

Answer 22

ethanolic concentrated NH3, heat in sealed tube (to prevent ammonia from escaping)

Question 23

Why should excess ammonia be used during nucleophilic substitution of halogenoalkanes with NH3 to form amines?

Answer 23

During the reaction, different amines can be formed when excess halogenoalkane is used, causing the yield of the primary amine to decrease, and makes the separation difficult. To obtain primary amine as the main product, excess ammonia should be used.

Question 24

Why are halogenoarenes less susceptible to nucleophilic substitution than halogenoalkanes? (2)

Answer 24

1) the lone pair of electrons on the halogen atom delocalises into the benzene ring. As a result, there is partial double bond character (transfer of pi electrons) in the C–X bond, so its bond length is shorter and bond strength is stronger. Thus, the C–X bond is very difficult to break.
2) Sterically, the rear side of the C–X bond is blocked by the benzene ring. The pi electron cloud of the benzene ring will also repel the lone pair of electrons of an incoming nucleophile, making the attack of nucleophile difficult.

Question 25

What reactions do halogenoarenes undergo?

Answer 25

The halogenoarenes undergo electrophilic substitution on the benzene ring similar to other arenes. However, they are much less susceptible to electrophilic substitution as the halogen substitution is deactivating. A higher temperature is needed.

Question 26

What is the general method of distinguishing between the type of halogen substituents present in halogenoalkanes? (3 steps)

Answer 26

1) Add NaOH and heat to form halogen ion.
2) Add excess dilute HNO3 to remove unreacted OH- so that brown Ag2O ppt will not be formed in the next step.
3) Add AgNO3 (aq) and observe the colour of ppt formed.

Question 27

What colour is the precipitate of AgCl after undergoing the distinguishing test and what is its solubility in NH3 solution?

Answer 27

White.

Dissolves in dilute NH3 (aq)

Question 28

What colour is the precipitate of AgBr after undergoing the distinguishing test and what is its solubility in NH3 solution?

Answer 28

Cream.

Dissolves in concentrated NH3 (aq)

Question 29

What colour is the precipitate of AgI after undergoing the distinguishing test and what is its solubility in NH3 solution?

Answer 29

Yellow.

Insoluble even in concentrated NH3 (aq)

Question 30

How can we compare rate of formation of AgX precipitates during the distinguishing test? (2 steps)

Answer 30

1) add 1cm3 of organic compound into a test tube.

2) add 1cm3 of ethanolic AgNO3 and place the test tube in a hot water bath.

Question 31

What is the respective rate of formation for 3 different AgX compounds? Why?

Answer 31

AgCl: 5-8 min
AgBr: 3-5min
AgI: almost immediately
Bond strength decreases in the order C–Cl > C–Br > C–I. Since the C–X bond broken is the rate determining step, the stronger the C–X bond, the higher the activation energy of the rate determining step, and hence the slower the rate.

Question 32

What are halogenoalkanes commonly used for?

Answer 32

Refrigerants, aerosol propellants and fire extinguishers. Especially when inertness is required, fluoroalkanes are more useful.

Question 33

How does the use of CFCs (chlorofluorocarbons) deplete the ozone layer?

Answer 33

The long lifespan of CFCs allow them to diffuse into the upper stratosphere, where strong UV radiation cleaves the weaker C–Cl bond to produce chlorine radicals, which catalyses the decomposition of ozone into oxygen. In the process, chlorine radicals are regenerated while removing oxygen atoms needed to make more ozone.

Question 34

How can we protect the ozone layer? (2)

Answer 34

1) reduce use of CFCs

2) use substitutes such as hydrocarbons, hydrofluorocarbons or fluorocarbons.