In BF3 Boron atom has 6e can we call it an
electrophile?
Secondly why LiAlH4 is more reactive than NaBH4 as reducing
agent.
Yes, the BF3 molecule is an electrophilic Lewis acid (with the B
atom being the dative bond acceptor). The term "Lewis acid" refers
to its propensity to accept dative bonds, while the term
"electrophile" refers to its propensity to seek electrons, so both
terms are closely related, but not technically the same. In
addition, the terms "nucleophile" and "electrophile" are by
convention, favored for use specifically in Organic Chemistry,
while the terms "Lewis base" and "Lewis acid" are overarching
archetypes across all branches of Chemistry. For Cambridge A level
Chemistry purposes, exam-smart students are advised to combine both
terms (ie. any and all relevant terms or keywords for all questions
in general) to secure the marks, even if Cambridge is usually
reasonable and will accept either term.
For 2 closely related reasons. Firstly, because Al has a larger
atomic radius than B, and thus polarizability of the electron bond
pairs of the H- hydride ions increase with increasing distance from
the central 'cationic' atom, as predicted by Coloumb's law, and
also due to increasing shielding effect from larger number of inner
shell electrons. Secondly, the lattice dissociation enthalpy for
the covalent bonds with the hydride ions becomes less endothermic
(ie. bond weakens) as bond length increases from BH4- to AlH4-, due
to less effective overlap of more diffused electron shell orbitals
involved in forming the sigma bonds. For both reasons above,
concordantly and consequently, LiAlH4 is a significantly more
effective delivery agent of H- hydride ion nucleophiles, compared
to NaBH4.