Reactions of Nitriles (RCN) [R] to form 1° Amines (RNH2)

R&C: LiAlH4 in dry ether H2, Ni as catalyst, heat

14: Halogen Derivatives Flashcards by Isabelle Wee

Halogen Derivatives

Definition

Halogenoalkanes (RX) ie. Alkyl Halides: ≥ 1 H atom is replaced by an X atom

Halogenoarenes (ArX) ie. Aryl Halides: ≥ 1 H atom of ⏣ is replaced by an X atom & is bonded directly to ⏣

where Ar = ⏣; X = F, Cl, Br or I

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Halogenoalkanes (RX)

Classification

With respect to (wrt.) C atom bonded to X,

1 R group: 1°
2 R groups: 2°
3 R groups: 3°

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Halogenoalkanes (RX)

Physical Properties - Boiling Point (b.p)

b.p of RX ↑ than corresponding Alkane
b.p ↑ as size of R group increases for RX with the same X atom
b.p ↑ as the size of X increases for RX with the same R group

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Halogenoalkanes (RX)

Physical Properties - Solubility

Since RXs are polar compounds ∵ polar C-X bond,

poor solubility in H₂O ∵ unable to interact via H bonds
soluble in organic compounds eg. CCl₄, hexane ∵ able to interact via id-id dipole forces

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Halogenalkanes (RX)

Physical Properties - Density

Compared to H₂O,

R-F & mono R-Cl < H₂O
poly R-Cl, R-Br & R-I > H₂O

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Synthesis of RX

Halogenation of Alkanes

R&C: X₂, UV light/ Heat

Mechanism: FRS

where X = Cl, Br

High chance of poly-sub. ∴ Limited X₂ for mono-sub

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Synthesis of RX

Halogenation of Alkenes

Mono-add
R&C: Dry HX_(g), room temp.
Di-add
R&C: X₂ in CCl₄, room temp.

Mechanism: EA

where X = Cl, Br, I

Vicinal dihalides ie. RX with 2 adjacent yet different X atoms

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Synthesis of RX

Nucleophilic Substitution of ROH - Chlorination

R&C:

Dry HCl_(g), (ZnCl₂), heat
conc. HCl, heat
Anhydrous PCl₃, heat
Anhydrous PCl₅, room temp.
Anhydrous SOCl₂, room temp.

Mechanism: NS

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Synthesis of RX

Nucleophilic Substitution of ROH - Bromination

R&C:

conc. H₂SO₄, KBr → Dry HBr_(g) produced in situ, heat
Anhydrous PBr₃, heat
Anhydrous SOBr₂, heat

Mechanism: NS

Eqn for in situ rxn is OUT OF SYLLABUS
SOBr₂ is less stable & widely used than SOCl₂

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Synthesis of RX

Nucleophilic Substitution of ROH - Iodination

R&C:

conc. H₃PO₄, KI → Dry HI_(g) produced in situ, heat
Red P, I₂ → Anhydrous PI₃ produced in situ

PI₃ not sufficiently stable for storage

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Reactions of RX

Nucleophilic Substitution to form ROH

R&C:
NaOH_(aq)/ KOH_(aq), heat (under reflux)

OH_(g) acts as Nucleophile in aqueous medium

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Reactions of RX

Nucleophilic Substitution to form RCN

R&C:
Ethanolic KCN, heat (under reflux)

Mechanism: NS

Step-up Rxn

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Reactions of Nitriles (RCN)

[R] to form 1° Amines (RNH₂)

R&C:

LiAlH₄ in dry ether
H₂, Ni as catalyst, heat

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Reactions of RCN

Hydrolysis

Acidic Medium
R&C: H₂SO_4(aq), heat
Alkaline Medium
R&C: NaOH_(aq), heat

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Reactions of RX

Nucleophilic Substitution to form RNH₂

R&C: Excess conc. ethanolic NH₃, heat in a sealed tube

Low chance for poly-alkylation. ∴ Limited NH₃ for poly-alkylation

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Reactions of RX

Nucleophilic Substitution to form Esters

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R&C: Carboxylate ion, heat

where HC chain can be of any length

Reactions of RX/ ArX

Nucleophilic Substitution to form Ethers

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R&C:

Alkoxide ion, heat
Phenoxide ion, heat

where HC chain can be of any length

Reactions of RX

Dehydrohalogenation ie. Elimination to form Alkenes

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R&C: Ethanolic NaOH/ KOH, heat

Condition: H atom adjacent to X atom

OH^- acts as a Base in alcoholic medium

Nucleophilic Substitution (NS)

S_N1 Mechanism

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Formation of carbocation (Rate-determining ie. slow step)

Heterolytic fission of C-X bond where ^δ+C is sp³-hybridised
Formation of trigonal planar carbocation wrt to (+)vely-charged, sp²-hybridised C atom

Nucleophilic attack on carbocation (Fast step)

Nu attacks from either side of the plane with equal chance
Inversion & retention of stereochemical configuration in equal proportions
Racemisation occurs ie. racemic mixture formed ie. equal amounts of enantiomers produced

Rate Equation: rate = k[RX]

where S = substitution, N = nucleophilic, 1 = unimolecular rxn

Unimolecular rxn = 1 rxt particle involved in rate-determining step

Nucleophilic Substitution (NS)

S_N1 Mechanism - Factors

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Strength of C-X bond (C-F > C-Cl > C-Br > C-I)
ē weaker ē C-X bond, ē faster ē ROR.
∴ R-F (no reaction), R-Cl < R-Br < R-I
Stability of carbocation intermediate (CH₃⁺ < 1° R⁺ < 2° R⁺ < 3° R⁺)
ē lower E_a of formation of carbocation, ē faster ē formation of a more stable carbocation intermediate, ē faster ē ROR
∵ ↑ no. of ED R groups → ↑ dispersal of (+)ve charge → ↑ stable ē carbocation intermediate
∴ CH₃X < 1° RX < 2° RX < 3° RX

Italics determine if rxn proceeds via S_N1/ S_N2

C-F bond is too strong to be broken no matter under normal lab conditions/ prolonged heating under reflux as part of hydrolysis

Nucleophilic Substitution (NS)

S_N2 Mechanism

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Backside nucleophilic attack
- Heterolytic fission of C-X bond
- Nu attacks ^δ+C from side directly opposite X atom
Transition State (T.S)
- Simultaneous partial formation of C-Nu & partial cleavage of C-X bond
- Gradual lose & gain of (-)ve charge respectively on both Nu & X result in δ- on both
Inversion of stereochemical configuration

Rate Equation: rate = k[RX][Nu]

where S = substitution, N = nucleophilic, 2 = bimolecular rxn

Bimolecular rxn = 2 rxt particle involved in rate-determining step

Nucleophilic Substitution (NS)

S_N2 Mechanism - Factors

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Strength of C-X bond (C-F > C-Cl > C-Br > C-I)
ē weaker ē C-X bond, ē faster ē ROR.
∴ R-F (no reaction), R-Cl < R-Br < R-I
Steric hindrance of R groups (no. of R groups around ^δ+C)
ē ↑ e^- cloud size of groups surrounding ^δ+C, ē ↑ hindrance of backside attack
∴ CH₃X > 1° RX > 2° RX > 3° RX

Italics determine if rxn proceeds via S_N1/ S_N2

C-F bond is too strong to be broken no matter under normal lab conditions/ prolonged heating under reflux as part of hydrolysis

Halogenoarenes (ArX)

Physical Properties

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Sooty, luminous flame upon combustion
Colourless liquids
Insoluble in H₂O
Denser than H₂O

Synthesis of ArX

Halogenation

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R&C: X₂, AlX₃/ FeX₃/ Fe as catalyst

Mechanism: ES

Distinguishing Tests | Hydrolysis

**Test:** 1. Add NaOH_(aq) to separate test tubes containing R-Cl, R-Br & R-I and heat. [Hydrolyses RX to produce X^-] 2. Cool the mixture 3. Acidify the mixture with HNO_3(aq) [Removes excess NaOH which can react with AgNO₃] 4. Add AgNO_3(aq) to the mixture [Produces ppt. of AgX_(s)] **Observations:** R-Cl: White ppt. of AgCl R-Br: Pale cream ppt. of AgBr R-I: Yellow ppt. of AgI | Btwn R-Cl, R-Br & R-I; RX & ArX/ other functional groups

Distinguishing Tests | Nucleophilic Substitution + Acid-Base + Precipitation

**Test:** Add ethanolic AgNO₃ and warm the mixture **Observations:** Coloured ppt. of AgX for RX

14: Halogen Derivatives Flashcards

(26 cards)