[a student]

hi
as you know CH bond can do hyper conjugation but CD can't what is the reason of this event?

[SniperDogs]

I'm taking my first semester of ochem... and I haven't studied hyperconjugation yet.

All I know is D is a heavier isotope of Hydrogen.  Hope that helps.

[Schrödinger]

Bond strength.

C-D > C-H. So, it is tougher for C-D to break.
Maybe for H the effect is more pronounced, but I am not sure if there is NO hyperconjugation at all in case of D.

[savy2020]

as you know CH bond can do hyper conjugation but CD can't what is the reason of this event?

C-D can undergo hyper conjugation but it's effect is less than that of C-H due to

Bond strength.
C-D > C-H. So, it is tougher for C-D to break.

You can see this for reference. Chk from pg15.

[orgoclear]

well even C-C can show hyperconjugation ..
but its effect is even less pronounced
See Chp 4 of Carey (in the stability of carbocations section)

[g-bones]

C-D bonds can hyperconjugate, it is less favorable though.  this is the foundation of a secondary kinetic isotope effect.  the vibrational frequency of C-D bonds is much smaller and it prefers to maintain that.  hyperconjugation weakens bonds by removing some electron density to help stablize the carbocation.  this is much more unvaforable with C-D.  hope this helps.