07 Alkenes and Alkynes

The logic of multiple-bond reactions

Alkenes contain a carbon–carbon double bond, and alkynes contain a carbon–carbon triple bond. Their electron-rich pi bonds can form new bonds with atoms or groups through addition. As addition proceeds, an alkene double bond becomes a single bond, while an alkyne triple bond may become a double bond or a single bond.

To predict a product, track three features:

• Regioselectivity: which group attaches to each end of an unsymmetrical multiple bond.

• Stereoselectivity: whether groups add to the same face (syn), opposite faces (anti), or without a specific facial preference.

• Reaction extent: whether an alkyne reacts once, reacts twice, or is reduced completely.

The reagent and reaction pathway determine these outcomes; no single orientation or stereochemical rule applies to every reaction.

Alkene additions: orientation and stereochemistry

A common electrophilic-addition pathway begins when the alkene reacts with an electrophile to form a carbocation. A nucleophile then bonds to the positively charged carbon. The pathway tends to favor the orientation that forms the more stable carbocation, explaining the usual Markovnikov orientation of hydrogen-halide addition. For example, adding HBr to propene usually gives 2-bromopropane.

A carbocation is planar, so a nucleophile can attack either face. These reactions are generally not stereospecific, and rearrangements may occur.

There is a reagent-specific exception: HBr in the presence of peroxides reacts by a radical pathway and gives predominantly the anti-Markovnikov product. For propene, this product is 1-bromopropane. In standard introductory reactions, this peroxide effect is characteristic of HBr; do not assume it applies to HCl or HI.

Halogen addition takes a different pathway. An alkene forms a bridged halonium ion, which a halide attacks from the opposite face. This produces anti addition and a vicinal dihalide. When water is present, it can act as the nucleophile instead, forming a halohydrin. The hydroxyl group usually attaches to the more substituted carbon, and the addition remains anti.

Alkene hydration depends on the pathway. Acid-catalyzed hydration with H2O, H+ usually gives a Markovnikov alcohol, but its carbocation intermediate can rearrange. Oxymercuration–demercuration, using 1. Hg(OAc)2, H2O; 2. NaBH4, also gives a Markovnikov alcohol while avoiding rearrangements associated with a free carbocation.

In hydroboration–oxidation, hydrogen and boron add to the same face, with boron attaching preferentially to the less substituted carbon. Oxidation replaces the carbon–boron bond with a carbon–oxygen bond while retaining its orientation. The net result is syn, anti-Markovnikov hydration without a carbocation intermediate. For example, 1-butene gives 1-butanol.

Catalytic hydrogenation with H2 and a metal such as Pd, Pt, or Ni adds hydrogen to the same face of an alkene on the catalyst surface, reducing it to an alkane.

Takeaway: First identify the pathway. Carbocation pathways commonly favor Markovnikov products and may rearrange; halonium-ion opening gives anti addition; hydroboration gives syn, anti-Markovnikov addition.

Alkyne additions and hydration

Alkynes undergo many of the same additions as alkenes, but their triple bond can react twice. Reagent amount and conditions therefore determine whether a reaction stops at an alkene or continues further.

• Hydrogen halides: One equivalent of HX usually gives a vinyl halide. Excess can give a geminal dihalide, with both halogens on the same carbon. Addition to a terminal alkyne usually follows Markovnikov orientation.

• Halogens: One equivalent of Br2 or Cl2 gives a dihaloalkene, commonly by anti addition. Excess halogen can add again to form a tetrahalide.

• Mercury-catalyzed hydration: H2SO4, H2O, HgSO4 gives Markovnikov addition to form an enol, which then converts into a carbonyl compound. A terminal alkyne gives a methyl ketone. For example, CH3CH2C≡CH gives 2-butanone.

• Hydroboration–oxidation: This route gives anti-Markovnikov hydration, followed by conversion of the enol into a carbonyl compound. With a bulky borane, a terminal alkyne gives an aldehyde. For example, 1-butyne gives butanal.

An enol has both a carbon–carbon double bond and an OH group attached to one of the double-bonded carbons. Its conversion into a carbonyl compound is tautomerization, a shift in the position of a hydrogen and a double bond. Unsymmetrical internal alkynes may produce mixtures of regioisomeric carbonyl compounds during hydration.

Takeaway: For an alkyne, track both regiochemistry and reagent equivalents. For hydration, identify the enol first, then determine the carbonyl product formed after tautomerization.

Selective reduction and oxidation

Hydrogenation is a reduction: hydrogen adds across a multiple bond. The reagent system determines whether an alkyne stops at an alkene and, if so, which alkene geometry forms.

• An alkene treated with H2 and a metal catalyst becomes an alkane; hydrogen adds syn.

• An alkyne treated with excess H2 and an active metal catalyst such as Pd/C becomes an alkane.

• An alkyne treated with H2 and a Lindlar catalyst becomes a cis alkene through partial syn reduction.

• An alkyne treated with Na or Li in liquid NH3 becomes a trans alkene through partial reduction.

For example, 2-butyne gives cis-2-butene with a Lindlar catalyst, but trans-2-butene with sodium in liquid ammonia. These conditions provide complementary ways to select alkene geometry. An ordinary active hydrogenation catalyst usually continues reducing the alkene intermediate to an alkane.

In organic chemistry, a carbon is generally oxidized when it gains a bond to a more electronegative atom such as oxygen, or loses a bond to hydrogen. It is reduced when it gains a bond to hydrogen or loses a bond to oxygen.

Takeaway: Choose the partial-reduction reagent according to the desired alkene geometry. An active catalyst with excess hydrogen can reduce the alkyne all the way to an alkane.

Oxidation, oxygen addition, and cleavage

Some oxidation reactions add oxygen-containing groups to alkenes; others split a multiple bond into smaller products.

• Epoxidation: A peroxyacid such as RCO3H transfers oxygen to an alkene, forming a three-membered cyclic ether called an epoxide. The concerted reaction preserves the relative arrangement of substituents across the double bond.

• Dihydroxylation: OsO4 adds two hydroxyl groups to the same face, producing a syn 1,2-diol. Alternatively, epoxidation followed by acid-catalyzed ring opening gives an anti (trans) 1,2-diol.

• Oxidative cleavage: O3 followed by a reductive workup such as Zn breaks the alkene double bond and converts each alkene carbon into a carbonyl carbon. Depending on substitution, products can include aldehydes and ketones. Strong conditions, such as hot acidic KMnO4, can further oxidize aldehydes to carboxylic acids; a terminal =CH2 carbon can be oxidized to CO2.

Strong oxidative cleavage also breaks an alkyne triple bond. Internal alkynes typically produce carboxylic acids, while cleavage of a terminal alkyne gives a carboxylic acid and CO2.

Takeaway: Determine whether oxidation adds oxygen-containing groups or cleaves the multiple bond. Workup conditions can change how far oxidation proceeds.

A product-prediction checklist

Use this sequence to predict products systematically:

1. Locate the double or triple bond and determine whether its two ends are equivalent.

2. Identify what the reagent does: addition, hydration, hydrogenation, oxidation, or cleavage.

3. For an unsymmetrical bond, assign groups using the regioselectivity of that particular reaction.

4. Add stereochemistry only when the mechanism or conditions specify it. Halonium-ion opening gives anti addition; hydroboration and catalytic hydrogenation give syn addition.

5. For alkynes, check reagent equivalents and conditions to determine whether addition stops at an alkene or continues to a saturated product.

6. If hydration forms an enol, convert it to the corresponding carbonyl compound by tautomerization.

Before finalizing a structure, check that it reflects the reagent, the reaction pathway, and the reaction extent.