halogen dervt

Question 1

why are b.p of halogenoalkanes higher than corresponding alkanes

Answer 1

both have simple molecular structure.

alkanes non-polar with only id-id interactions between molecules, while halogenoalkanes are polar with stronger pd-pd interatctions between molecules.

halogenoalkanes have larger electron cloud, greater distortion of electron cloud compared to alkanes, thus stronger id-id compared to alkanes

energy required to overcome id-id interactions larger for haloalkanes

Question 2

between different halogenoalkanes with similar number of c atoms, but different halogen atom, which compound (with halogen atom) is highest boiling point which is lowest and why?

Answer 2

I highest, followed by Br, then Cl, then F. Larger electron cloud size, larger extent of distortion of electron cloud, strength of id-id interactions between molecules increase, energy required to overcome id-id interactions increase.

Question 3

why are halogenoalkanes insoluble in water

Answer 3

halogenoalkane molecules cannot form intermolecular hydrogen bonds with water molecules, no strong interactions between halogenoalkane and water molecules. Insufficient energy is released to overcome strong hydrogen bonds between water molecules for mixing to occur.

Question 4

why are halogenoalkanes soluble in non-polar solvents

Answer 4

energy released from id-id interactions formed between non-polar alkyl groups of halogenoalkanes and non-polar solvent molecules is sufficient to overcome similar id-id interactions between halogenoalkane molecules and between non-polar solvent molecules

Question 5

why is free radical substitution not effective in producing a specific halogenoalkane

Answer 5

a mixture of monosubstituted and/or polysubstituted halogenoalkaes may be obtained

Question 6

why is using alkenes to produce halogenoalkanes not effective as well

Answer 6

a mixture of 2 products is formed when an unsymmetrical alkene reacts with HX (X halogen atom)

Question 7

define nucleophiles

Answer 7

electron pair donors attracted to an electron deficient atom or region of low electron density

Question 8

what happens when sn1 mechanism using an enatiomerically-pure chiral reactant

Answer 8

since carbocation is trigonal planar with respect to electron deficient carbon, nucleophile attacks carbocation from the top and the bottom with equal probability. If the carbon atom with positive charge becomes a chiral carbon after reaction, a racemic mixture is formed, optically inactive, as both mirror image enantiomers are formed in equal quantities.

Question 9

how are halogenoalkanes formed (optimal), give reaction

Answer 9

Using HX
R-OH +HX -> R-X + H2O

PCl5
R-OH +PCl5 -> R-Cl + POCl3 + HCl(g)

Question 10

compare the relative rates for sn2 mechanism for (methyl halides, 1st, 2nd, 3rd degree halogenoalkanes)

Answer 10

methyl-halides>1>2>3

Question 11

compare the relative rates for sn1 mechanism for (methyl halides, 1st, 2nd, 3rd degree halogenoalkanes)

Answer 11

3>2>1>methyl-halide

Question 12

why is sn1 mechanism favoured for tertiary halogenoalkanes

Answer 12

tertiary halogenoalkane gives stable tertiary carbocation intermediate as electron-donating alkyl groups disperse the positive charge on the carbocation intermediate. methyl/primary carbocation less stabilised

Question 13

why is sn2 mechanism, via penavalent transition state, is favoured for primary and methyl halogenoalkanes

Answer 13

methyl or primary halogenoalkane has no or only 1 alkyl group, bonded to partial postive carbon atom, allows for easy approach of the nucleophile. tertiary halogenoalkane has three alkyl groups, hinder approach of nucleophile to partial positive carbon atom.

Question 14

why does (CH3)3CCH2Cl undergo sn1 substitution mechanism even though it is a primary alkyl halide

Answer 14

steric hinderance due to bulky (CH3)3C- group, makes it difficult for nucleophiles to attack from opposite side of leaving group.

Question 15

why is chloromethylbenzene, which is also a primary alkyl halide, undergo sn1 substitution mechanism

Answer 15

resonance stabilisattion of benzyl carbocation by delocalisation of electrons from benzene ring

Question 16

reagents conditions for formation of alcohols (via nucleophilic sub), state nucleophile

Answer 16

NaOH(aq), heat under reflux, nucleophile is OH-

Question 17

reagents conditions for formation of nitriles, and state nucleophile

Answer 17

KCN in ethanol, heat under reflux, nucleophile is CN-

Question 18

why is formation of nitrile considered a step-up reaction

Answer 18

the carbon chain is extended by one carbon atom with each -CN group added

Question 19

how is a carboxylic acid formed from a nitrile (reagent, conditions, type of reaction)

Answer 19

heating with dilute acid, under reflux, acidic hydrolysis

Question 20

how is a carboxylate salt formed from a nitrile (reagent, conditions, type of reaction)

Answer 20

heating with dilute alkali under reflux, alkaline hydrolysis

Question 21

what are the 2 methods to form a primary amine from a nitrile (reagents, conditions, type of reaction for both)

Answer 21

H2(g), Ni catalyst, reduction

LiAlH4 (in dry ether), reduction

Question 22

reagents and conditions as well as nucleophile for formation of primary amines from halogenoalkanes

Answer 22

excess NH3 in ethanol heated under pressure, nucleophile is NH3

Question 23

what happens when LIMITED ammonia is used in excess in the formation of primary amines from halogenoalkanes

Answer 23

further nucleophilic substitution can take place to form a mixture of 1/2/3 degree amines and quaternary ammonium salt

Question 24

reagents and conditions for elimination reaction for halogenoalkanes

Answer 24

NaOH in ethanol, heat under reflux

Question 25

order reactivity of haloalkanes from slowest to fastest (R3C-F to R3C-I)

Answer 25

R3C-I fastest, R3C-F slowest, because C-I has longest bond length, weakest bond strength, BDE decreases

Question 26

characteristics of fluoroalkanes and fluorohalogenoalkanes

Answer 26

generally stable and unreactive, strong carbon-flourine bond. Good solvents with low boiling temperatures, and are non-flammable, non-toxic and odourless

Question 27

give applications for these fluoroalkanes and fluorohalogenoalkanes

Answer 27

fire extinguisher, refrigerant, aerosol propellant

Question 28

why are HCFCs seen as slightly better compared to CFCs

Answer 28

less powerful greenhouse gases than CFCs

Question 29

why do halogenoarenes not undergo nucleophilic substitution reactions, unless when subjected to very drastic and vigourous conditions

Answer 29

1) p-orbital of halogenatom overlaps side-on with p-orbitals of six carbon atoms in benzene ring. Lone-pair of electrons in p-orbital of halogen atom can delocalise into benzene ring to form delocalised electron cloud. Results in partial double bond character in C-X bond. more energy required to break stroger C-X boond and halogen atom very difficult to be displaced from benzene ring. Rear side of C-X bond in halogenoarenes is blocked by bulky benzene ring. pi electron cloud of benzene ring will repel lone pair of electrons of an incoming nucleophile, rendering attack of nucleophile difficult.

Question 30

how to test for halogen derivatives

Answer 30

add an equal volume o NaOH(aq) to halogenoalkane and heat in water bath. Cool the mixture and ass excess HNO3(aq). Finallly, add AgNO3(aq).

Question 31

why is aqueous HNO3 added in the test for halogenoalkanes

Answer 31

prevents unreacted hydroxide ions from reactng with Ag+ ions to give ppt, Ag2O.