14 Halogen Derivatives Flashcards
(26 cards)
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
Halogenoalkanes (RX)
Classification
With respect to (wrt.) C atom bonded to X,
- 1 R group: 1°
- 2 R groups: 2°
- 3 R groups: 3°
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
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
Halogenalkanes (RX)
Physical Properties - Density
Compared to H2O,
- R-F & mono R-Cl < H2O
- poly R-Cl, R-Br & R-I > H2O
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
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
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
Synthesis of RX
Nucleophilic Substitution of ROH - Bromination
R&C:
- conc. H2SO4, KBr → Dry HBr(g) produced in situ, heat
- Anhydrous PBr3
- SOBr2
Mechanism: NS
- Eqn for in situ rxn is OUT OF SYLLABUS
- SOBr2 is less stable & widely used than SOCl2
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
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
Reactions of RX
Nucleophilic Substitution to form RCN
R&C:
Ethanolic KCN, heat (under reflux)
Mechanism: NS
Step-up Rxn
Reactions of Nitriles (RCN)
[R] to form 1° Amines (RNH2)
R&C:
- LiAlH4 in dry ether
- H2, Ni as catalyst, heat
Reactions of RCN
Hydrolysis
Acidic Medium
R&C: H2SO4(aq), heat
Alkaline Medium
R&C: NaOH(aq), heat
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
Reactions of RX
Nucleophilic Substitution to form Esters
R&C: Carboxylate ion, heat
where HC chain can be of any length
Reactions of RX/ ArX
Nucleophilic Substitution to form Ethers
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
R&C: Ethanolic NaOH/ KOH, heat
Condition: H atom adjacent to X atom
OH- acts as a Base in alcoholic medium
Nucleophilic Substitution (NS)
SN1 Mechanism
- 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)
SN1 Mechanism - Factors
- 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 Ea 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 SN1/ SN2
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)
SN2 Mechanism
- 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)
SN2 Mechanism - Factors
- 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 SN1/ SN2
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
- Sooty, luminous flame upon combustion
- Colourless liquids
- Insoluble in H2O
- Denser than H2O
Synthesis of ArX
Halogenation
R&C: X2, AlX3/ FeX3/ Fe as catalyst
Mechanism: ES
