# Addition to conjugated dienes

$$\ce{CH2=CH-CH=CH2 ->[\ce{H2/Pt}][\pu{1 equiv}] ?}$$

I thought $$\ce{H2}$$ should add to one of the π bonds and form but-1-ene. But the answer is but-2-ene.

I couldn't understand this.

• Just read about $1,2$-addition vs. $1,4$-addition. – Soumik Das May 22 '19 at 6:35
• Okay. But isn't 1,4-addition only for electrophilic addition to conjugated dienes? But addition of H2 is by metal adsorption. – Sriya Jun 12 '19 at 3:04
• Yes, it is by metal adsorption. So, take a look at the simplest mechanism given here chem.libretexts.org/Bookshelves/Organic_Chemistry/… . Now at higher temperature there can be rearrangements in the binding of the alkene to the metal surface after addition of one hydrogen. That means the double bond can rearrange to form an intermediate in which the binding to metal surface is from the different carbon after rearrangement. And hence that can take hydrogen to form rearranged product. – Soumik Das Jun 13 '19 at 4:24
• Okay. Thank you. – Sriya Jul 5 '19 at 17:46