Question

Do propane, propene, and propyne belong to a homologous series? Explain.

Short answer

Propane, propene, and propyne do not belong to the same homologous series, as they have different molecular formulas and functional groups. Propane is an alkane with single bonds, propene is an alkene with a double bond, and propyne is an alkyne with a triple bond. Though they all have three carbon atoms and are hydrocarbons, their distinct molecular structures and functional groups place them in separate homologous series of alkanes, alkenes, and alkynes.

Step-by-step solution

Define a homologous series

A homologous series is a group of organic compounds that have the same general molecular formula, same functional group, and similar chemical properties. They have a gradual change in physical properties (such as boiling point and melting point) and show a regular gradation in their properties as a result of an increase in molecular size and mass.

Identify the molecular structures of propane, propene, and propyne

Propane (C₃H₈) is an alkane with the molecular formula CₙH₂ₙ₊₂, where n = 3. Propene (C₃H₆) is an alkene with the molecular formula CₙH₂ₙ, where n = 3. Propyne (C₃H₄) is an alkyne with the molecular formula CₙH₂ₙ₋₂, where n = 3.

Compare the molecular structures

Propane has a single bond between all carbon atoms, propene has a double bond between two carbon atoms, and propyne has a triple bond between two carbon atoms. They all have three carbon atoms and are hydrocarbons, but their molecular formulas are different, as are their functional groups: - Propane: Alkane (C-C single bonds) - Propene: Alkene (C=C double bond) - Propyne: Alkyne (C≡C triple bond)

Determine if propane, propene, and propyne belong to a homologous series

Since the molecular formulas and functional groups are different for propane, propene, and propyne, they do not belong to the same homologous series. They do, however, each belong to their own separate homologous series of alkanes, alkenes, and alkynes.

Key concepts

Organic Chemistry

Organic chemistry is a fascinating branch of chemistry that concentrates on the study of carbon-based compounds, which are the foundation of all living things on Earth. This field of science explores the structure, properties, composition, reactions, and synthesis of organic compounds and materials. It is a vast area that understands life at the molecular level, and it introduces us to diverse groups of compounds, many of which are used in our daily lives from medicines to plastics.

Central to organic chemistry is the versatile carbon atom, which has the unique ability to form strong covalent bonds with other carbons and a variety of other elements. These carbon bonds can be single, double, or triple, giving rise to an immense variety of compounds with different properties. Organic molecules can range from simple substances, like methane, to incredibly complex biopolymers and everything in between. Many of these organic compounds are categorized into various series based on similar structures and properties, known as homologous series.

Alkanes

Alkanes are the simplest family in organic chemistry and are sometimes called paraffins. They are hydrocarbons, which means they are compounds composed solely of carbon (C) and hydrogen (H) atoms. The remarkable feature of alkanes is their single covalent bonds that link the carbon atoms. This single bond is denoted by the chemical formula C-C.

Represented by the general molecular formula C_nH_2n+2, where 'n' represents the number of carbon atoms, alkanes are part of a homologous series with successive members differing by a CH₂ unit. From methane (CH₄) all the way up to eicosane (C₂₀H₄₂) and beyond, these compounds show gradual changes in their physical properties, like boiling and melting points. Alkanes are known for their low reactivity, which is why they are often used as fuels.

Alkenes

Moving on to alkenes, this group of organic compounds is characterized by containing one or more carbon-carbon double bonds, represented as C=C. The presence of this double bond is a distinctive feature of alkenes and is important in determining their reactivity. Alkenes conform to the general molecular formula of C_nH_2n, where 'n' is again the number of carbon atoms.

The double bonds not only define alkenes' chemical properties but also allow for geometric isomerism, meaning that compounds with the same molecular formula can have different physical properties due to a different arrangement of atoms in space. Ethene (C₂H₄), commonly known as ethylene, is the simplest alkene. This homologous series of alkenes is known for their role in the polymer industry, as double bonds can break and link chains together, forming plastics like polyethylene.

Alkynes

Alkynes distinguish themselves with carbon-carbon triple bonds, denoted by C≡C. This triple bond is not only a defining characteristic but also the basis for alkynes' chemical reactivity. Alkynes fit into the general molecular formula C_nH_2n-2, with 'n' representing the carbon count. While similar to alkanes and alkenes in being hydrocarbons, the presence of a triple bond in their structure often makes them more reactive.

For example, ethyne (C₂H₂), also known as acetylene, is the simplest member of the alkyne family. It is used industrially as a fuel and a chemical building block. Alkynes’ triple bonds have the ability to participate in various reactions, such as hydrogenation and addition reactions, making them valuable in organic synthesis. Despite having similar elemental compositions, alkanes, alkenes, and alkynes are part of different homologous series due to these chemical bonding differences.