EK mentions that the enol is less stable because C=C is weakened by the electronegativity of oxygen. However, isnt the -OH group electron donating? Why would an electron donating group then weaken the C=C bond?
electron density of lone pairs on the oxygen atom repels the pi electron density of the C=C bond which results in the unstability of C=C bond. A pi bond is electron rich bond and becomes unstable in the presence of electron donating groups.