You can substitute bicarbonate of soda for yeast, but you also need to include an acidic substance with it. A common pairing is lemon juice, but you can also use buttermilk. The bicarbonate of soda and acid combination is a 1:1 substitute for the yeast. This means if the recipe calls for 2 teaspoons of yeast, you'll substitute 1 teaspoon of bicarbonate of soda and 1 teaspoon of acid. You also don't need to let the dough rise or rest like you would with a typical yeast dough.