The addition of hydrogen to an alkene can occur in either a cis (same side) or trans (opposite side) manner. However, there are two common proofs that support the trans addition of hydrogen to an alkene. These proofs are based on experimental observations and stereochemical considerations. Here are the two proofs:
Syn Addition of Hydrogen: One of the proofs comes from the observation that the addition of hydrogen to an alkene occurs via a syn addition mechanism. Syn addition means that both hydrogen atoms add to the same face of the alkene molecule. This is supported by experimental evidence where it has been observed that the addition of hydrogen to an alkene occurs with the retention of stereochemistry. In other words, if the two substituents on the alkene are initially on the same side (cis configuration), they remain on the same side (trans configuration) after hydrogenation. This observation is consistent with a trans addition of hydrogen.
Anti-Markovnikov Rule: The second proof is based on the anti-Markovnikov rule, which states that in the addition of H2 to an alkene, the hydrogen atom adds to the carbon atom with fewer alkyl substituents. This rule is based on the regioselectivity of the reaction. When an alkene undergoes trans addition of hydrogen, the hydrogen atom adds to the carbon atom with fewer alkyl substituents, leading to the formation of a more stable alkane product. This observation supports the idea of trans addition of hydrogen.
These experimental and stereochemical observations provide evidence for the trans addition of hydrogen to an alkene.