Factors affecting Electron-availability in BondsFACTORS AFFECTING ELECTRON-AVAILABILITY IN BONDS AND AT
INDIVIDUAL ATOMS
In the light of what has been said above, any factors that influence
the relative availability of electrons (the electron density) in particular
bonds or at particular atoms in a compound will greatly affect its
reactivity towards a particular reagent; for a position of high electron
availability will be attacked with difficulty if at all by, for example,
®OH, whereas a position of low electron availability is likely to be
attacked with ease, and vice versa with a positively charged reagent.
A number of such factors have been recognised.- W
(i) Inductive effect
In a covalent single bond between unlike atoms the electron pair
forming the a bond is never shared absolutely equally between the
two atoms; it tends to be attracted a little more towards the more
electronegative atom Qj[the two. Thus in an alkyl halide
\ \8.* 8- \
—c<§3»ci —yc—ci -yc-*-c\
(XV); (JfVla) (XVlb)the electron density tends to be greater nearer chlorine than carbon
(XV) as the former is the more electronegative; this is generally
represented in classical formulae by (XVIa) or (XVlb). If the carbon
atom bonded to chlorine is itself attached to further carbon atoms,
tiro effect can be transmitted further:
c—c—c-*-c^-a
4 3 2 1
The effect of the chlorine atom's partial appropriation of the elec
trons of the carbon-chlorine bond is to leave Ct slightly electron-
deficient; this it seeks to rectify by, in turn, appropriating slightly
more than its share of the electrons of the a bond joining it to C 2 ,
and so on down the chain. The effect of C 1 on C 2 is less than the
effect of CI on Cu however, and the transmission quickly dies away
in a saturated chain, usually being too small to be noticeable beyond
Ca.
Most atoms and groups attached to carbon exert such inductive
effects in the same direction as chlorine, i.e. they are electron-with
drawing, owing to their being more electronegative than carbon, the