My cake crumbled on the top and took very long time to cook This time I didn't have much white sugar, so I added half cup brown and half cup white sugar to my carrot cake.

I observed that the cake crumbled on the top with large cracks, and took very long time to cook as compared to previous same cake in which I had used just white sugar.

The change that I did to the cake was to add half cup brown sugar. The taste was same as before.

Can brown sugar be the culprit here?
Brown sugar contains molasses, which is acidic, and probably threw off the acid-base balance in the cake. It also contains more water than white sugar, but neither contain much, so that probably didn't matter.

Did the cake rise as expected? I'm guessing “no”, which is probably what changed the baking time.

Quickly looking it up, between ¼ tsp and ½ tsp of baking soda would neutralize the brown sugar, if you feel like trying again.

Other possibilities:


your baking powder or soda is too old; how long ago was the last cake? Baking powder especially has a limited shelf-life.
mistakes (oven temperature set wrong, measured wrong)