In organic chemistry, an acetal is a functional group with the connectivity R2C(OR')2. Here, the R groups can be organic fragments or hydrogen, while the R' groups must be organic fragments, not hydrogen. The two R' groups can be equivalent to each other or not. An acetal is formed from and convertible to two alcohol molecules and an aldehyde or ketone, but acetals have substantially different chemical stability and reactivity as compared to the analogous carbonyl compounds. The central carbon atom has four bonds to it, and is therefore saturated and has tetrahedral geometry.