The weaker the base, the better the leaving group. With your question, -OCH3 is a larger molecule (more electrons from the methyl donating group) and will more easily donate electrons (think kinetics), but it is also a weaker base than -OH.
Alcohols have hydroxyl groups (OH) which are not good leaving groups. Why not? Because good leaving groups are weak bases, and the hydroxide ion (HO–) is a strong base. This will convert the alcohol into an alkyl bromide or alkyl chloride, respectively, and halides (being weak bases) are great leaving groups.
The alkoxide and the hydroxides aren't good leaving groups. The water molecule now attached is a good leaving group (oxygen has a positive charge). Nucleophilicity does not determine the suitability of a leaving group. A group is said to be a good leaving group when it can leave as a relatively stable ion.
after all its the conjugate base of water. and when we turn on a tap in the kitchen, we aren't usually trying to get a strong acid to drink ! But water itself, H2O, is a good leaving group, since it is the conjugate base of H3O+, which is a strong acid.
The leaving group ability decreases as the basicity of the nucleophile increases. Since the basicity increases in the order: Br−<Cl−<CH3COO−<HO−<H−, therefore, their leaving group ability decreases in the reverse order, i.e., Br−>Cl−>CH3COO−>HO−>H−.
Leaving group (LG; nucleofuge): An atom or group of atoms which breaks away from the rest of the molecule, taking with it the electron pair which used to be the bond between the leaving group and the rest of the molecule. Water is the leaving group in this SN1 reaction.
The rate of SN2 reactions goes primary > secondary > tertiary. The “big barrier” to the SN1 and E1 reactions is carbocation stability. The rate of SN1 and E1 reactions proceeds in the order tertiary > secondary > primary.
Iodine is a better leaving group than other halogen atoms due to its larger size. Due to larger size, charge density decreases and it becomes stable. So, its a better leaving group.
5. For SN2, The Rate Of Reaction Increases Going From Tertiary To Secondary To Primary Alkyl Halides. For SN1 The Trend Is The Opposite. For the SN2, since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).
The difference between the eliminations of alcohols and amines in acidic solution is the poorer leaving group ability of ammonia than that of water (remember, ammonia is a stronger base; therefore a poorer leaving group.)
N2 is best leaving group among given. Best leaving group are self detachable. N2 is self detachable.
Strong nucleophiles:
| VERY Good nucleophiles | HS–, I–, RS– |
|---|
| Good nucleophiles | Br–, HO–, RO–, CN–, N3– |
| Fair nucleophiles | NH3, Cl–, F–, RCO2– |
| Weak nucleophiles | H2O, ROH |
| VERY weak nucleophiles | RCO2H |
The more available the electrons, the more nucleophilic the system. Within a group in the periodic table, increasing polarisation of the nucleophile as you go down a group enhances the ability to form the new C-X bond and increases the nucleophilicity, so I- > Br- > Cl- > F-.
It's a much stronger acid, in other words, and therefore its conjugate base (water, H2O) is much weaker. In other words, by adding acid, we've made it a better leaving group. This is a general phenomenon, by the way – the conjugate acid will always be a better leaving group.
You won't see the term “electrophile” as much as you will “nucleophile”, but you will see “electrophilic site”, which refers to the reactive, electron poor part of the substrate. Leaving Groups: Leaving groups are the part of the molecule that is booted off during a substitution or elimination reaction.
