[lazipeter]

Why CH3CH2Cl, having no vacant orbital on the halogenated carbon, can still accept nucleophilic attack from OH- to perform a SN2 reaction??

But CH3CH3 cannot accept nucleophilic attack from OH-???

thanks!!

[Zainb]

Because the chlorine is a good leaving group but the hydrogen is not.

[movies]

Well, a better explanation is that the C–Cl sigma* anti-bonding orbital is relatively low-lying.  When a nucleophile approaches there is a positive overlap between the Nu lone pair and the sigma* that in turn weakens the C–Cl bond and ultimately leads to displacement.  In the C–H case, the sigma* orbital is higher in energy, so the the overlap is not as favorable when a nucleophile approaches.

[lazipeter]

How about CCl4? Its Cl atom also cannot be displaced by OH-!
another example is NF3, will not react with OH- owing to the lack of vacant orbital.

thx!!

[dunno260]

Fluorine is a poor leaving group (ie has a much higher lieing sigma* orbital).  NF₃ has vacant orbitals, but they aren't sufficiently low lieing for the hydroxide anion to displace it.  In the case of CCl₄, the chlorine atoms are rather bulky so you can consider it somewhat like doing an S_N2 reaction on t-butyl chloride.  The antibonding orbital is present, but its difficult for the hydroxide anion to access it to the steric bulk of the substituents.  In the case of CHCl₃, if you react it with hydroxide you won't get nucleophilic substitution but rather will see deprotonation to form trichloromethide (CCl₃^-).

Every element and compound has vacant orbitals.  Though we don't ever consider that hydrogen say has d orbitals, it does, they are just of such a high energy that they don't matter to the chemistry.  Those orbitals have to exist because we can see them in the emission spectrum of hydrogen when you excite it and the electron relaxes to the ground state. 

[lazipeter]

O I see
Thank you!

[movies]

CCl₄ is indeed reactive with some nucleophiles despite its bulk.  For example, it reacts quite readily with PPh₃, although the nucleophile attacks a Cl atom instead!