Class 12 Chemistry Haloalkanes Notes

Class 12 Chemistry Haloalkanes Notes
CHEMISTRY • UNIT 8

Haloalkane (Alkyl halide)

Exact-typing reconstruction from the supplied 26-page handwritten source.

Original Scanned PDF – View Notes

Open PDF Full Screen
Open PDF in Google Drive

Unit 8 — Haloalkane (Alkyl halide)

Haloalkane are the halogen derivative of alkane in which at least one hydrogen is replaced by halogen atom. Eg

CH3–CH3−H / +X→CH3–CH2–X
AlkaneHaloalkane

Eg

CH3–CH2–Br

Bromoethane
(ethyl bromide)

CH3–CH2–CH2–Br

1-bromopropane
propyl bromide

CH3 — CH — CH3
       |
       Br

2-Bromopropane
isopropyl bromide

CH3–CH2–CH2–CH2–Br

1-bromobutane
n-butyl bromide

CH3 — CH — CH2 — CH3
       |
       Br

2-bromobutane
iso butyl bromide

       CH3
        |
CH3 — C — CH3
        |
        Br

2-bromo-2-methylpropane
tert-butyl bromide

Types of Haloalkane

[Unreadable in source]

A) On the basis of number of halogen atom

i) Monohaloalkanes

Having only one halogen atom.

CH3–CH2–Br   —   Bromoethane

ii) Dihaloalkane

Having two halogen atom. Dihaloalkane are also two type.

a) Geminal dihalides (Gem)
       Br
        |
CH3 — CH — Br

(1,1) dibromoethane

b) Vicinal dihalides (vic)
CH2 — CH2
 |      |
 Br     Br

1,2 dibromoethane

iii) Polyhaloalkane

Having more than two halogen atom.

  • CHCl3 — chloroform
  • CHI3 — Iodoform
  • CHBr3 — bromoform

B) On the basis of halogen attached carbon atom

i) Primary (1°) haloalkane

The haloalkane in which halogen atom is attached with primary carbon atom is known as primary haloalkane.

CH3–CH2–Br

Bromoethane

CH3–CH2–CH2–Br

Bromopropane

ii) Secondary (2°) haloalkane

The haloalkane in which halogen atom is attached with secondary carbon atom is known as secondary haloalkane.

CH3 — CH — CH3
       |
       Br

2-Bromopropane

iii) Tertiary (3°) haloalkane

The haloalkane in which halogen atom is attached with tertiary carbon atom is known as tertiary haloalkane.

       CH3
        |
CH3 — C — CH3
        |
        Br

2-bromo-2-methylpropane
(tert-butyl bromide)

Isomers in Haloalkanes

The organic compound having same molecular formula but different in structure is known as structural isomers. Haloalkane contain two types of structural isomers.

1) Chain isomers

The isomers in which only carbon chain is different is known as chain isomers.

CH3–CH2–CH2–CH2–Br

1-Bromo butane

     CH3
      |
CH3—CH—CH2—Br

1-bromo-2-methylpropane

ii) Position isomers

The isomers in which only position of halogen atom is different is known as position isomers.

CH3–CH2–CH2–Br

1 bromopropane

CH3 — CH — CH3
       |
       Br

2-bromopropane

General preparation of haloalkane

1) From alkane: [halogenation]

Alkane react with halogen gives haloalkane.

CH3–CH3 + F2dark→CH3–CH2–F + HF
CH3–CH3 + Cl2sunlight→CH3–CH2–Cl + HCl
CH3–CH3 + Br2hν / Δ→CH3–CH2–Br + HBr
CH3–CH3 + I2conc. HNO3 / HIO3→CH3–CH2–I + HI
Note: Reactivity   F2 > Cl2 > Br2 > I2.

Probability of release of H-atom from alkane during halogenation:

3° > 2° > 1°
CH3–CH2–CH3 + Cl2→CH3–CH2–CH2–Cl (45%) + CH3–CH(Cl)–CH3 + HCl

Hence these reaction are not favourable to prepare specific haloalkane because alkane when react with halogen produce mixture of haloalkane.

2) From alkene

Alkene react with halogen acid gives haloalkane.

CH2=CH2 + HBr→CH3–CH2–Br
etheneBromoethane
CH3–CH=CH–CH3 + HI→CH3–CH2–CH(I)–CH3
2 Butene2-Iodobutane

If alkene is unsymmetrical then addition of halogen acid carried out by markonikov’s rule.

Markonikov’s rule

When unsymmetrical substrate (alkene and alkane) react with unsymmetrical reagent (HCl, HBr, HI, H-OH) then -ve part of reagent attached with that multiple bonded carbon atom which have less no. of H-atom and +ve part goes with higher no of hydrogen atom.

CH3–CH=CH2 + HBr→CH3–CH(Br)–CH3

2 Bromopropane

Anti-markonikov’s rule / kharash effect / peroxide effect

When unsymmetrical substrate react with unsymmetrical reagent in presence of organic peroxide then -ve part of reagent attached with that multiple bonded carbon atom which had higher no. of hydrogen and +ve part goes with less no of hydrogen.

CH3–CH=CH2 + HBrROOR→CH3–CH2–CH2–Br
propene1 Bromopropane
NOTE: HI and HCl are not suitable for anti-markonikov’s rule.

3) From Alcohol

a) Reaction with halogen acid

Reactivity of halogen acid: HI > HBr > HCl > HF

Reactivity of alcohol: 3° > 2° > 1°

Alcohol react with halogen acid in presence of conc. HCl and anhydrous ZnCl2 (Lucas reagent) gives haloalkane.

CH3–CH2–OH + HClanhy. ZnCl2 / Δ→CH3–CH2–Cl + H2O
CH3–CH2–OH + HBranhy. ZnCl2→CH3–CH2–Br + H2O
ethyl alcohol
CH3–CH2–OH + HIroom temp→CH3–CH2–I + H2O
CH3–CH2–OH + HF→CH3–CH2–F + H2O
       CH3
        |
CH3 — C — OH
        |
       CH3

2-methyl propan 2-ol
tert-butyl alcohol

       CH3
        |
CH3 — C — Cl
        |
       CH3

2 chloro-2-methyl propane

b) Reaction with phousphorous halide

3CH3–CH2–OH + PCl3→3CH3–CH2–Cl + H3PO3
CH3–CH2–OH + PCl5→CH3–CH2–Cl + POCl3 + HCl
Chloroethane
3CH3–CH2–OH + PBr3Red (P) + Br2→3CH3–CH2–Br + H3PO3
Bromoethane
3CH3–CH2–OH + PI3Red (P) + I2→3CH3–CH2–I + H3PO3
Iodoethane

c) Action with thionyl chloride (SOCl2) [Doorzen’s rxn]

Alcohol react with thionyl chloride (SOCl2) in presence in pyridine (C6H5N) gives chloro alkane.

CH3–CH2–OH + SOCl2pyridine / Δ→CH3–CH2–Cl + SO2↑ + HCl↑

chloroethane

NOTE: Bromo alkane and iodo alkane cannot be prepared by this process because (SOBr2) thionyl bromide and iodide cannot be exist.

Properties

  • lower members are gases and higher are liquid then solid.
  • Haloalkane are insoluble in water but soluble in organic solvent.

Chemical properties

1) Wurtz Synthesis

Haloalkane when heated with sodium metal in presence of dry ether gives alkane with double no of carbon.

2CH3–CH2–Br + 2Nadry ether / Δ→CH3–CH2–CH2–CH3 + 2NaBr
BromoethaneButane
2(CH3)2CH–Br + 2Nadry ether / Δ→(CH3)2CH–CH(CH3)2 + 2NaBr
2 Bromopropane2,3 dimethylbutane
CH3 — CH — CH3
       |
       Br

2 Bromopropane

CH3 — CH — CH3
       |
CH3 — CH — CH3

2,3 dimethyl butane

2) Dehydrohalogenation (elimination rxn) formation of alkene (Rxn with alc. KOH)

Haloalkane when react with alc KOH then elimination of hydrogen and halogen occurs from adjacent carbon gives alkene.

CH3–CH2–Clalc. KOH→CH2=CH2 + KCl + H2O
chloroethaneethene
CH3–CH2–CH2–Bralc. KOH→CH3–CH=CH2 + KBr + H2O
1-Bromopropanepropene

If there is chances of formation of two types of alkene then that alkene is consider as major product which is obtained by the removal of higher degree of hydrogen (Saytzeff’s rule).

CH3–CH2–CH(Br)–CH3alc. KOH→CH3–CH2–CH=CH2 (1 Butene)   /   CH3–CH=CH–CH3 (2-Butene)

3) Action with aq. KOH or NaOH (formation of alcohol)

Haloalkane react with aqueous KOH or NaOH gives alcohol.

CH3–CH2–Br + KOH→CH3–CH2–OH + KBr
Bromoethaneethanol
CH3–CH(Br)–CH3 + NaOH→CH3–CH(OH)–CH3 + NaBr
2-Bromopropanepropan-2-ol (isopropyl alcohol)
NOTE: Haloalkane react with aqueous KOH gives alcohol because aqueous KOH produce hydroxyl ion (OH−) but alc. KOH gives alkene because it could not produce OH− ion.

4) Action with KCN or NaCN (formation of Nitrile / cyanide?)

Haloalkane react with alcoholic KCN or NaCN gives alkane nitrile.

CH3–CH2–Br + KCN→CH3–CH2–CN + KBr
ethyl cyanide / propane nitrile
CH3–CH(Br)–CH3 + KCN→CH3–CH(CN)–CH3 + KBr
2 bromopropane2-methyl propane nitrile

Cyanide can be converted into carboxylic acid, amide and primary amine.

CH3–CH2–CN
H2SO4+
Complete hydrolysis→CH3–CH2–COOH
propanoic acid
concn HCl / H2O
partial hydrolysis→CH3–CH2–CONH2
propanamide
LiAlH4 / Na/C2H5OH
H2/Ni / Ni/Pt / Ni/Pd→CH3–CH2–CH2–NH2
propan amine

5) Reaction with alc. AgCN

Haloalkane react with alc. AgCN gives isocyanide.

CH3–CH2–Br + AgCN→CH3–CH2–NC + AgBr

(ethyl isocyanide)

Isocyanide undergoes reduction gives secondary amine.

CH3–CH2–NC + 4[H]LiAlH4 / Na / C2H5OH / Ni / Pt / Pd→CH3–CH2–NH–CH3

[Unreadable in source]

NOTE: question
CH3–Br + KCN / NaCN → CH3–CN
CH3–Br + alc. AgCN → CH3–NC

Because KCN and NaCN are ionic and produce CN− ion but AgCN is covalent nature.

6) Reaction with KNO2 or NaNO2 (Potassium nitrite)

Haloalkane react with KNO2 or NaNO2 gives alkyl nitrite.

CH3–CH2–Br + KNO2→CH3–CH2–O–N=O + KBr
(ethyl bromide)(ethyl nitrite)
K+ [O–N=O]−   →   K+ + [O–N=O]−

7) Reaction with AgNO2

Haloalkane react with alc. AgNO2 gives nitro alkane.

CH3–CH2–Br + AgNO2→CH3–CH2–NO2 + AgBr
Bromoethane(Nitroethane)
NOTE — Question
CH3–Br + KNO2 / NaNO2 → CH3–O–N=O   (Nitrite)
CH3–Br + AgNO2 (alc) → CH3–NO2   (Nitro)

Because KNO2 and NaNO2 are ionic compound but AgNO2 is covalent.

Imp • Ambident Nucleophile

The nucleophile having double attaching site is called ambident nucleophile.

8) Rxn with sod. pot alkoxide (williamson’s synthesis (etherification))

Primary haloalkane react with sodium or potassium alkoxide gives symmetrical as well as unsymmetrical ether. These reaction are called etherification reaction.

CH3–CH2–Br + NaO–CH3→CH3–CH2–O–CH3 + NaBr
sod. methoxideMethoxy ethane
CH3–CH2–Br + NaO–CH2–CH3→CH3–CH2–O–CH2–CH3 + NaBr
sod. ethoxide(ethoxyethane)
NOTE: 2° and 3° haloalkane are not suitable for this reaction.

9) Action with Ag2O

Haloalkane react with Ag2O at 250°C produce symmetrical ether.

2CH3–CH2–Br + Ag2O250°C→CH3–CH2–O–CH2–CH3 + AgBr
Bromoethaneethoxy ethane (diethyl ether)

Haloalkane when heated with Ag2O at 350°C produce alkene.

2CH3–CH2–Br + Ag2O350°C→2CH2=CH2 + 2AgBr + H2O

ethene

10) Reduction reaction

Haloalkane undergoes reduction in presence of catalyst gives alkane.

CH3–CH2–Br + 2[H]Sn-HCl / Zn-HCl / Fe-HCl
LiAlH4 / Ni-Pt / NiAl→CH3–CH3 + HBr

ethane

Q: [Unreadable in source]

Grignard reagent (R–MgX) — Imp topic

Alkyl magnesium halide are called grignard reagent. They can be prepared by heating alkyl halid with magnesium metal in presence of dry ether.

R–X + Mgdry ether / Δ→R–MgX
Alkyl halideGrignard reagent
CH3–Br + Mgdry ether / Δ→CH3–MgBr
methyl bromidemethyl magnesium bromide

Application of grignard reagent

1) Synthesis of alkane

Grignard reagent when react with H2O / NH3 or alkyl halide gives alkane.

CH3–CH2–MgBr + H–OH→CH3–CH3 + Mg(OH)Br
CH3–CH2–MgBr + H–NH2→CH3–CH3 + Mg(NH2)Br
CH3–CH2–MgBr + CH3–Br→CH3–CH2–CH3 + MgBr2

ethylmagnesium bromide

2) Synthesis of carboxylic acid

Grignard reagent react CO2 gives addition product which on acidic hydrolysis gives carboxylic acid.

CH3–MgBr + CO2→CH3–C(=O)–OMgBrH2O / H+→CH3–COOH + Mg(OH)Br
addition product • ethanoic acid / acetic acid
CH3–CH2–MgBr + CO2→CH3–CH2–C(=O)–OMgBrH2O / H+→CH3–CH2–COOH + Mg(OH)Br

propanoic acid

3) Synthesis of alcohol

Grignard reagent react with formaldehyde (methanal) gives addition product which on acidic hydrolysis produce primary (1°) alcohol.

CH3–MgBr + H–C(=O)–H→CH3–CH2–OMgBrH2O / H+→CH3–CH2–OH + Mg(OH)Br
methanalethanol (ethyl alcohol)
CH3–CH2–MgBr + H–C(=O)–H→CH3–CH2–CH2–OMgBrH2O / H+→CH3–CH2–CH2–OH + Mg(OH)Br
propanol (propyl alcohol)

Grignard react with aldehyde except formaldehyde and followed by acidic hydrolysis gives secondary

[2°] alcohol

CH3–MgBr + CH3–CHO→CH3–CH(OMgBr)–CH3H2O / H+→CH3–CH(OH)–CH3 + Mg(OH)Br
(acetaldehyde) ethanalpropan-2-ol (isopropyl alcohol)
CH3–CH2–MgBr + CH3–CHO→CH3–CH(OMgBr)–CH2–CH3H2O / H+→CH3–CH(OH)–CH2–CH3 + Mg(OH)Br
butan-2-ol (isobutyl alcohol)

Grignard reagent react with keton then followed by acidic hydrolysis gives tertiary [3°] alcohol.

CH3–MgBr + CH3–CO–CH3→(CH3)3C–OMgBrH2O / H+→(CH3)3C–OH + Mg(OH)Br

(2-methyl propan-2-ol) (tert-butyl alcohol)

CaOCl2 + H2O – Bleaching powder

Polyhaloalkane

The haloalkane having more than two halogen atom are called polyhaloalkane.

CHCl3

Chloroform
(1,1,1) trichloromethane

CHBr3

Bromoform
tribromomethane

CHI3

Iodoform
triiodomethane

Laboratory preparation of chloroform (trichloromethane) — Imp

Principle

In lab chloroform is prepared by heating paste of bleaching powder with ethyl alcohol or acetone.

CaOCl2 + H2O→Ca(OH)2 + Cl2

(paste of bleaching powder)

A) From ethyl alcohol

i) Oxidation

CH3–CH2–OH + Cl2Δ→CH3–CHO + 2HCl
ethanolacetaldehyde (ethanal)

ii) Chlorination

CH3–CHO + 3Cl2Δ→CCl3–CHO + 3HCl

Trichloroacetaldehyde • Chloral • Trichloroethanal

iii) Hydrolysis

2CCl3–CHO + Ca(OH)2Δ→2CHCl3 + (HCOO)2Ca
chloroform • calcium formate

Overall rxn — Imp

2CH3–CH2–OH + Ca(OH)2 + 8Cl2Δ→2CHCl3 + (HCOO)2Ca + 10HCl
ethyl alcohol • paste of bleaching powderchloroform • Ca formate

B) From acetone

i) Chlorination

CH3–CO–CH3 + 3Cl2Δ→CCl3–CO–CH3 + 3HCl
propanone (acetone)Trichloroacetone

ii) Hydrolysis

CCl3–CO–CH3 + Ca(OH)2Δ→2CHCl3 + (CH3COO)2Ca
chloroform • Ca acetate / Ca ethanoate

Overall rxn

CH3–CO–CH3 + Ca(OH)2 + 6Cl2Δ→2CHCl3 + (CH3COO)2Ca + 6HCl
acetone • paste of bleaching powderchloroform • Ca acetate

Properties of chloroform — Imp

  • It is colourless sweet smelling compound.
  • It is good solvent for oil, fat and waxes.
  • It is insoluble in H2O but soluble in organic solvent like ether, alcohol etc.
  • The vapour of chloroform causes temporary unconsciousness so it is used as general anaesthetic.

Chemical Properties

1) Action with air — Imp

In presence of sunlight chloroform react with atmospheric oxygen gives extremely poisonous carbonyl chloride which is also called phosgene gas.

2CHCl3 + O2sunlight→2COCl2 + 2HCl

Carbonyl chloride (phosgene)

If chloroform is filled up to the brim of bottle, then all air is displaced and phosgene cannot be formed.

Little amount of ethyl alcohol is mixed with chloroform during its packing to convert the poisonous phosgene into non poisonous.

2) Action with conc. HNO3

Chloroform react with conc. HNO3 gives nitro chloroform which is called chloropicrin. Chloropicrin is an insecticide which is used as a tear gas.

CCl3–H + HO–NO2→CCl3–NO2 + H2O

Nitro chloroform • Chloropicrin

3) Action with acetone

Chloroform react with acetone gives chloretone which is a hypnotic drugs and used as a sleep inducing drugs.

CCl3–H + CH3–CO–CH3→(CH3)2C(OH)–CCl3

chloretone • (1,1,1) trichloro-2-methyl propane-2-ol

4) Action with Ag powder

Chloroform when heated with Ag powder gives acetylene gas.

2CHCl3 + 6AgΔ→HC≡CH + 6AgCl

acetylene gas • ethyne

2CHCl3 + 6AgΔ→CH≡CH + 6AgCl

ethyne • acetylene gas

2CHI3 + 6AgΔ→CH≡CH + 6AgI

ethyne

5) Reaction with aq. alkali (KOH, NaOH)

Chloroform react with aq. alkali gives formic acid which further react with aq alkali gives pot. / sod. formate.

CHCl3 + 3KOH→HC(OH)3 + 3KCl−H2O→HCOOHKOH→HCOOK + H2O
formic acid / methanoic acid • pot. formate

6) Formation of isocyanide (Carbylamine rxn)

Primary amine react with chloroform in presence of alkali gives isocyanide. These rxn is used as a carbyl amine test.

CH3–CH2–NH2 + CHCl3 + 3KOH→CH3–CH2–NC + 3KCl + 3H2O
ethanamineethyl isocyanide

Aniline → phenyl isocyanide

NH₂ Aniline + CHCl₃ + 3KOH → NC phenyl isocyanide + KCl + 3H₂O

7) Reimer Tiemann rxn

Phenol react with chloroform in presence of alcoholic alkali gives o-hydroxy benzaldehyde which is also called (salicylaldehyde).

OH Phenol + CHCl₃ + 3KOH (alc) → OH CHO o-hydroxybenzaldehyde + 3KCl + 3H₂O

If CCl4 is used in state of chloroform gives o-hydroxy benzoic acid also called salicylic acid.

OH + CCl₄ + 3KOH → OH COOH o-hydroxybenzoic acid • salicylic acid + 3KCl + 3H₂O

8) Reduction

In acidic medium

CHCl3 + 2[H]Zn/HCl→CH2Cl2 + HCl
Trichloromethanedichloromethane

In Neutral medium — Imp

CHCl3 + 6[H]Zn/H2O→CH4 + 3HCl
chloroformmethane

9) Action with AgNO3 Solution (aq AgNO3)

Chloroform is covalent compound and cannot be produce chloride (Cl−) ion. so AgNO3 soln does not gives white ppt with pure chloroform.

AgNO3 soln + CHCl3 pure→No rxn.

But impure chloroform (CHCl3 + HCl) react with AgNO3 solution gives white ppt of AgCl.

CHCl3 + HClAgNO3→AgCl + HNO3
impure chloroformwhite ppt.

Uses of chloroform

  • It is used to prepare chloretone, chloropicrin etc.
  • It is used as good organic solvent for
  • fats, oils, waxes, resins etc.
  • It is used as general anaesthetic agent but nowadays remove.
KEEP LEARNING View All Posts

Discussion

Share a helpful question, idea, or explanation with other students.

Leave a Comment

Write a clear question, answer, or helpful explanation.
Your email will not be published.

Download Our Offline App

Study class-wise notes even when internet is not available. Get the app from Play Store.

Nepal eNotes offline app preview
Get it on Google Play