Which β-hydrogen goes?

A haloalkane with its β-hydrogens highlighted: remove one with alcoholic KOH and the alkene forms; the sim shows substitution count on the C=C and the major/minor split; aqueous KOH switches to substitution.

Remove the β-hydrogen from 2-bromopentane that gives the major product (pent-2-ene)

Pent-1-ene is the major product / aqueous KOH eliminates.

• The carbon bearing the halogen is the α-carbon and its neighbour is the β-carbon. Heating a haloalkane that has a β-hydrogen with alcoholic KOH removes H from the β-carbon and X from the α-carbon to give an alkene: β-elimination, or dehydrohalogenation.
• Zaitsev (Saytzeff) rule (1875): when more than one alkene can form, the major product is the one with more alkyl groups on the doubly bonded carbons. 2-Bromopentane gives mainly pent-2-ene.
• Substitution and elimination compete. A bulky nucleophile tends to act as a base and pull off a proton. Primary halides favour SN2; secondary halides go SN2 or elimination depending on the strength of the base or nucleophile; tertiary halides go SN1 or elimination depending on the stability of the carbocation or of the more substituted alkene.
• Aqueous KOH mainly substitutes (alcohol); alcoholic KOH mainly eliminates (alkene).
• With certain metals, most organic chlorides, bromides and iodides give organometallic compounds, which hold a carbon-metal bond.
• Grignard reagents, RMgX, form from a haloalkane and magnesium in dry ether; Victor Grignard reported them in 1900 and shared the 1912 Nobel Prize in Chemistry with Paul Sabatier.
• In RMgX the C–Mg bond is covalent but highly polar, with carbon drawing electrons from magnesium; the Mg–X bond is essentially ionic.
• Grignard reagents react with any proton source to give hydrocarbons; even water, alcohols and amines are acidic enough (RMgX + H₂O → RH + Mg(OH)X). Moisture must therefore be kept out, which is why dry ether is used; the reaction is also a way to turn a halide into a hydrocarbon.
• Wurtz reaction: an alkyl halide with sodium in dry ether gives an alkane with twice the number of carbon atoms (2RX + 2Na → R–R + 2NaX).

Teaching this idea? Put the simulation on your own page or LMS with this code.