Alkenes and Alkynes: Synthesis
- Dihalides: Both 1,1 and 1,2 dihalides can be turned into alkynes with a strong base.
- In some cases, there is a need to create an alkyne from an alkene. This can be done by the halogenation of the alkene to a 1,2-dihalide, which is then dehalogenated.
From Other Alkynes
- An alkyne can be made longer by the reaction of an alkynide anion and a primary alkyl chloride or bromide.
- Cis Alkenes from Alkynes: Hydrogen gas is mixed with the alkyne over a palladium catalyst that has been poisoned with lead acetate and quinoline, precipitated onto a calcium carbonate support. This catalyst, known as the Lindlar catalyst, is used to stop the reduction of the alkyne at the alkene level, and prevent full reduction to the alkane.
The result is a cis alkene because the hydrogen is adsorbed onto the surface of the catalyst. The result is that when the hydrogen is added to the alkyne, it is added simultaneously to the same side of the molecule.
- Trans Alkenes from Alkynes: Lithium is dissolved in liquid ammonia, and then the alkyne is added. Because the hydrogen atoms are added independently of each other, a trans addition occurs because it is more stable than the cis isomer due to steric strain. See the mechanism section for more details.
- Tertiary Alcohols: An acid is used to remove the hydroxyl group, and a hydrogen atom on the most substituted carbon follows, to make the most substituted double bond in an E1 reaction.
- Primary and Secondary Alcohols: POCl3 in pyridine is used to create a double bond via an E2 reaction. See the alcohol mechanism page (here) for more details.
Dehydrohalogenation usually takes place via an E1 or E2 elimination depending on how substituted the halide is, as well as the reaction conditions. Stronger bases tend to promote E2 elimination, while weaker bases and nucleophiles promote E1 reactions, as well as substitution reactions. For more information, see the organohalide reactions page.
- Primary Alcohols: This elimination requires a strong base to occur. For the best results, a strong base needs to be used. Generally, potassium tert-butoxide is used, because it is both very basic and a bad nucleophile due to its large size. A polar aprotic solvent, such as DMSO or HMPA is used, because it is good at solvating the potassium ion, which makes the butoxide more basic.
- Secondary Alcohols: Once again, a strong base produces the best results. However, in unsymmetrical organohalides, a mixture of products is almost always formed. Generally speaking, one product will be the major product, but there will usually be a considerable amount of the minor product(s). Zaitsev's Rule states that in these reactions, the most stable alkene produced will normally be the major product. This means that the more highly substituted alkene will predominate, and the trans isomer will predominate over the cis isomer. For more information on this rule, visit the Reaction Types page here.
- Tertiary Alcohols: An E2 elimination occurs when a base is used, just like for the primary and secondary alcohols. However, when aqueous alcohol is used, and the mixtures is gently heated, an E1 elimination (along with and SN1 reaction) occurs.
Synthesis of Aldehydes and Ketones
- Wittig Reaction: a phosphorus ylide (sometimes called a phosphorane) is formed by the reaction of triphenylphosphine (a nucleophile) on primary or secondary alkyl halide. Butyllithium is used to remove a hydrogen from the carbon attached to the phosphorus atom:
The negatively charged carbon atom from the ylide is a strong nucleophile that attacks the carbonyl group in a ketone or aldehyde. The oxygen binds to the phosphorus in the triphenylphosphine, and an alkene is formed. See the mechanism page for details.
The Wittig reaction is useful because it produces only the E and Z confomers of one isomer, as opposed to elimination reactions, which produce a much greater mix. For example, 2-methylcyclohexanone reacts with the phosphorane to produce 1-methyl-2-methylenecyclohexane, a disubstituted alkene.
- Hofmann Elimination: An amine group in a molecule is methylated with CH3I to produce an ammonium salt. A gentle base, normally AgO2, along with water and heating, is used to carry out an elimination. Unlike typical E2 elimination reactions, Hofmann eliminations generally give the less substituted alkene.
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