Halogen Derivitives Flashcards
How to determine degree of substitution of HX?
Number of alkyl or aryl groups bonded to the C bonded to the halogen.
Explain the difference in boiling point between halogenoalkanes and their corresponding alkanes
RX has higher bp than alkanes with same number of C atoms
1. pd-pd present in polar RX molecules
2. stronger id-id because of more electrons giving rise to higher polarisability
Bp of R-X down the group?
R-Cl<R-Br<R-I
From Cl to I, no. of electrons increase —> more polarisable electron cloud—> stronger id-id —> more energy required to overcome —> higher bp
Cannot use pd-pd discussion because this only affects when size of molecule is roughly the same
Solubility of RX
Insoluble in water, soluble in organic solvents
Energy released from the formation of … between … (in)sufficient to overcome energy absorbed from the breaking of … between …
Preparation of RX from OH (all possible reagents and conditions)
PCl5
PCl3/PBr3/PI3, heat
SOCl2, heat
HX (g), heat (if HCl, need ZnCl2 catalyst)
Preparation of RX from OH (preferred reactions)
For RCl, PCl5
For RBr, PBr3, heat
For RI, HI(g), heat
For preparation of RX from OH, why must PI3 be prepared in situ? (And what is in situ)
In situ = prepared at site of reaction; because very unstable.
Large atomic size = less effective orbital overlap between I and P = longer and weaker bond which require little energy to overcome (easily break down esp with heat already provided during the reaction)
For preparation of RX through alchohol with HCl(g), what catalyst is needed and why?
ZnCl2. H-Cl less reactive with alchohols because of stronger bond than H-Br and H-I. Electronegativity decreases down the group, meaning that it has the most polar and hence strongest bond. Hence it is the weakest nucleophile.
With ZnCl2, it can form a complex with alchohol (RO + HZnCl2), increasing hydroxyl group’s leaving potential (because more electronegative) so that Cl can displace it (RCl + Zn(OH)Cl) –> can react to reform catalyst (ZnCl2) + H2O
Ways of preparing RX
NS of alchohol
EA of HX to alkenes
Explain the types of reactions RX undergo
NS. Electron-deficient C in C-X bond has partial positive charge, allowing it to be attacked by a nucleophile.
NS of RX (w OH)
+ Types of compounds that can be formed
NaOH/KOH (aq), heat
From RX: alcohol
From dihalides: aldehyde/ketone
From trihalides: carboxylic acid
Formation of nitrile
KCN/NaCN (alc), heat (step up reaction)
Hydrolysis of nitrile
Acid hydrolysis (H2SO4 (aq), heat)
Base hydrolysis (NaOH (aq), heat)
Reduction of nitrile
Product: Primary amide
LiAlH4 in dry ether
OR H2, Ni Catalyst, heat
OR H2, Pd/Pt Catalyst
Why must LiALH4 be in dry ether?
LiAlH4 is highly soluble in ether, but reacts violently with water to produce H2 gas, so we want to maintain a dry and inert environment as much as possible.
Formation of amine
Replace X with NH2
Excess NH3 (alc) OR limited RX, heat in sealed tube/under high pressure
Strength of amine as nucleophiles
More substituted = stronger nucleophile cos more electron donating I effect.
Dehydrohalogenation of HX
NaOH/KOH (alc)
alc, because OH in water acts as a nucleophile (lewis base) but OH in alc acts as a bronsted base which reacts with HX to give KX + H2O
Distinguishing tests for RX
- Heat RX with NaOH (aq) - alkaline hydrolysis
- Acidify with HNO3 (aq) - prevent formation of brown ppt (AgOH or Ag2O) in step 3
- Add AgNO3 (aq) - form ppt
Cream: RBr
Yellow: RI
White: RCl
Why cannot use HCl or H2SO4 instead to acidify?
For RX distinguishing test step 2
Introduces new anions into the system that also form white ppt with Ag+.
Why does medium for formation of nitrile (from RX) need to be alcohol?
Ammonia has different behaviour in different mediums. In alcohol it’s a nucleophile but in water it acts like a base (similar to OH behaviour)
Amine may also react with water by donating electron to H+
