SN1 reaction

It looks to me like you've explained it yourself.

The least reactive compound has a Chlorine bounded to a carbon which is directly connected to a phenyl group. When the Chlorine is removed, the carbocation formed will be stabilised by the positive inductive effect of the phenyl ring and it's delocalised pi-system consisting of six electrons.

The second compound has the carbon bonded to three methyl groups. It will form a tertiary carbocation - a carbocation stabillised by the positive inductive effect of three methyl groups. This offers less stability than a phenyl group so this species would be more reactive.

The most reactive compound will produce a primary carbocation - a carbocation bounded to a single methyl group. The positive inductive effect here is smaller and therefore the carbocation is less stable and more reactive.

Positive inductive effect is the idea of electron density being fed towards a region of positive charge, thus stabilising it. 6 pi-electrons in a phenyl ring constitutes a large positive inductive effect.

In an SN1 reaction the compound C6H5C(CH3)2 Cl forms the positively charged intermediate C6H5C(CH3)2+. The extra stabilisation of this intermediate provided by the phenyl ring, as described earlier, (compared to (CH3)3) makes its formation more favourable.

Positive inductive effect is the idea of electron density being fed towards a region of positive charge, thus stabilising it. 6 pi-electrons in a phenyl ring constitutes a large positive inductive effect.

I think I understand now your confusion.

The positive inductive effect is used to describe the electron releasing effect of groups via sigma bonds. It's effect is usually explained by different groups on a phenyl ring. Hence the confusion (I think!). It doesn't have to be on a phenyl ring.

Ther phenyl group feeds electrons towards the positive charge on the carbon via the sigma bond therefore there is a positive inductive effect. It's an inductive effect FROM the phenyl TOWARDS the carbon atom. Remember this is where the + charge is, not on the ring. i.e. C6H5C+

Another way of thinking this. Imagine two phenyl bonded together via one bond. Each phenyl ring would have a positive inductive effect on the other, stabilising any positive charges formed on either ring.

Yes exactly ('phenyl').

So the phenyl can be electron donating and electron withdrawaing???? When to use electron donating or withdrawing??

When is it election withdrawing? Do you have an example?

I think this induction effect is confusing things and leading us (or me) the wrong way.

The phenyl containing compound most readily reacts via SN1 because the resulting + ion is best stabilised.

Why is is best stabilised? The pi system of the phenyl ring can interact with carbon cation dispersing the the charge and stabilising it. This pi interaction is not an inductive effective, which uses sigma bonds. This is why phenyl gives greater stabilisation over CH3.

The pi electron system is also involved in the benzoic acid example you gave. it's effectively a 'charge disperser'.

I hope that makes more sense.

I still confused. Can you explain it in a more simple manner?

C6H5C(CH3)Cl is most likely to react via SN1 as it contains Ph and this is the best stabilising group/electron donator in C+ intermediate.

As you pointed out, Ph can act as electron donator with neighbouring + charge and electron withdrawer with neighbouring - charge.