When debentures are issued at par and are redeemable at a premium, the loss on such issue is debited to :

When debentures are issued at par but are safely promised to be redeemed at a premium, the premium on redemption is a future liability. Under prudence, this expected loss is recorded at the time of issue with the following entry:
$$\text{Debenture Application \& Allotment A/c}\text{Dr.}$$
$$\text{Loss on Issue of Debentures A/c}\text{Dr.}$$
$$\text{To \% Debentures A/c}$$
$$\text{To Premium on Redemption of Debentures A/c}$$
Thus, the loss is debited to Loss on issue of debentures account.