Question

Alkenes and alkynes are characterized by their ability to undergo rapid, complete ________ reactions, by which other atoms attach themselves to the carbon atoms of the double or triple bond.

Short answer

Alkenes and alkynes are characterized by their ability to undergo rapid, complete addition reactions, by which other atoms attach themselves to the carbon atoms of the double or triple bond.

Step-by-step solution

Identify the characteristic reaction of alkenes and alkynes

Alkenes and alkynes are unsaturated hydrocarbons that contain double and triple carbon-carbon bonds, respectively. These multiple bonds make them chemically reactive. They typically undergo a type of reaction called an addition reaction, where other atoms or groups of atoms attach to the carbon atoms in the double or triple bonds, breaking the multiple bonds.

Fill in the blank

Now that we know the type of reaction alkenes and alkynes undergo most, we can fill the blank with the correct term: Alkenes and alkynes are characterized by their ability to undergo rapid, complete \(\underline{\text{addition}}\) reactions, by which other atoms attach themselves to the carbon atoms of the double or triple bond.

Key concepts

Alkenes

Alkenes are a class of unsaturated hydrocarbons that contain at least one carbon-carbon double bond. This double bond is the defining feature of alkenes, making them quite reactive in chemical reactions. The simplest alkene is ethene (C₂H₄), commonly used in the production of plastics like polyethylene.

The double bond in alkenes consists of one sigma (σ) bond and one pi (π) bond. The pi bond is relatively weaker compared to the sigma bond, which is why alkenes are prone to reactions that break the π bond without disturbing the σ bond, such as addition reactions.

• Addition reactions involve the breaking of the double bond to attach additional atoms or molecules, such as hydrogen (in hydrogenation) or halogens like chlorine and bromine (in halogenation).
• This results in converting the alkene into an alkane or other saturated compounds.

These reactions are significant industrially and biologically, leading to the synthesis of many important products.

Alkynes

Alkynes represent another category of unsaturated hydrocarbons but with at least one carbon-carbon triple bond. The simplest alkyne is ethyne, also known as acetylene (C₂H₂).

The triple bond consists of one sigma bond and two pi bonds, giving alkynes a linear structure. This kind of bonding affects their reactivity. Like alkenes, alkynes also participate in addition reactions, but due to their triple bond, they can add extra molecules in such reactions.

• The addition reactions for alkynes often start with the conversion of the triple bond into a double bond, and with further reactions, into a single bond.
• An example is the hydrogenation of alkynes to alkanes, where hydrogen atoms add across the triple bond.

Alkynes are widely used in chemical synthesis and play an important role in the creation of plastics, synthetic routes, and other organic compounds.

Unsaturated Hydrocarbons

Unsaturated hydrocarbons, including alkenes and alkynes, are compounds that contain double or triple carbon-carbon bonds. These multiple bonds make these hydrocarbons more reactive compared to their saturated counterparts, like alkanes.

The term "unsaturated" refers to the presence of fewer hydrogen atoms bonded to carbon atoms than is possible in fully saturated hydrocarbons. This gives them some unique properties and applications:

• They typically engage in addition reactions, where atoms or groups are added to the carbon atoms across the double or triple bonds.
• These reactions increase the hydrogen or other atom content, converting them to a more saturated state.

Unsaturated hydrocarbons are also fundamental in various synthetic processes and the manufacture of essential materials like plastics, rubbers, and fuels. Their reactive nature makes them invaluable as starting materials in many chemical reactions.