Chemistry - Reactivity

I would like to clarify please - usually the reactivity of halides decrease down Group 7 right? So why does the halide C-X (in a halogenoalkane) increase down the group? Also what is the number one factor/priority that is given when determining the reactivity of a compound?

When we're reacting with a halogenoalkane, perhaps a nucleophilic substitution with a cyanide ion to form a nitrile, notice how we're breaking the C-X bond (where X is your halide)

If we can break that C-X bond easily, then it's more reactive because the mechanism is more likely to proceed.

Compare C - F and C - Br.
Bromine is less reactive than fluorine. That's because it has a large atomic radius and lots of shielding. This makes it far less electronegative.

C - F is very polar. The fluorine atom is very small so the bond is very short, and the electrons are going to be strongly attracted to the fluorine atom. This forms a strong bond.

Contrast that with the C - Br bond which is much longer, less polar and more weaker; this makes it easier to break. That's why it makes it more reactive.

Ahh I see thank you so much! So would you say that the only exception to the usual trend is with this carbon bond? As in are there any other cases in which the rule you explained above applies?

I can't really think of any other cases off the top of my head, but I do want to illustrate the fact that there's a whole host of factors that come into play when determining the reactivity of a compound...

For instance, we've just the bond enthalpy for the halogenoalkanes is the driving factor.

The other user, I <3 work, highlighted bond length, which to be honest is inseperable from bond enthalpy. Longer bond lengths are weaker due to the weaker electrostatic attraction, so the enthalpy change to break it is lower.


Then you've got bond polarity which is the driving factor for acids and bases. Because the HCl bond is more polar than HBr, it makes it more ready to ionise and donate a proton as a BL acid.
If you haven't covered this, then whoops sorry.

I guess this is just one of them things you got to learn.
Trust, when you start learning more in chemistry and you come across concepts like charge density and polarisation, you'll realise how you can't say there's this one rule you apply to all compounds that determines it's reactivity

I really appreciate this, you have explained the concept very well! I'm only at AS stage, but my problem is I can never proceed with my revision when I don't understand something along the way. That's why I result to finding out information beyond my level. But thank you once again, I understand now how reactivity can be grouped into different categories when determining it for a certain compound.