Alkanes are also called saturated hydrocarbons because they have the maximum number of hydrogen atoms attached to each carbon atom. This means that alkanes are relatively unreactive and stable. They undergo combustion (burning) in the presence of oxygen to produce carbon dioxide and water. They also react with halogens (such as chlorine and bromine) to form halogenated alkanes, which are used as solvents and refrigerants.
The physical properties of alkanes depend on their molecular size and shape. As the number of carbon atoms increases, the boiling point, melting point and density of alkanes also increase. This is because larger molecules have stronger intermolecular forces (such as van der Waals forces) that hold them together. Alkanes with straight-chain structures tend to have higher boiling points than those with branched-chain structures, because straight chains can pack more closely together. Alkanes are insoluble in water but soluble in non-polar organic solvents.
Alkenes
Alkenes are hydrocarbons that have at least one double bond between carbon atoms. They have the general formula CnH2n, where n is the number of carbon atoms. For example, ethene (C2H4), propene (C3H6) and butene (C4H8) are alkenes with two, three and four carbon atoms, respectively.
Alkenes are also called unsaturated hydrocarbons because they have fewer hydrogen atoms than alkanes with the same number of carbon atoms. This means that alkenes are more reactive and less stable than alkanes. They undergo addition reactions, in which a molecule is added across the double bond. For example, alkenes can be hydrogenated (addition of hydrogen) to form alkanes, halogenated (addition of halogens) to form dihalogenated alkanes, or hydrated (addition of water) to form alcohols.
The physical properties of alkenes are similar to those of alkanes, except that alkenes have lower boiling points and melting points than alkanes with the same number of carbon atoms. This is because the double bond reduces the symmetry of the molecule and weakens the intermolecular forces. Alkenes are also insoluble in water but soluble in non-polar organic solvents.
Alkynes
Alkynes are hydrocarbons that have at least one triple bond between carbon atoms. They have the general formula CnH2n-2, where n is the number of carbon atoms. For example, ethyne (C2H2), propyne (C3H4) and butyne (C4H6) are alkynes with two, three and four carbon atoms, respectively.
Alkynes are also called unsaturated hydrocarbons because they have fewer hydrogen atoms than alkenes with the same number of carbon atoms. This means that alkynes are even more reactive and less stable than alkenes. They undergo similar addition reactions as alkenes, but they can also undergo other reactions, such as polymerization (joining of many molecules to form a large molecule) and oxidation (loss of electrons or increase in oxidation state).
The physical properties of alkynes are similar to those of alkenes, except that alkynes have lower boiling points and melting points than alkenes with the same number of carbon atoms. This is because the triple bond reduces the symmetry of the molecule and weakens the intermolecular forces even more. Alkynes are also insoluble in water but soluble in non-polar organic solvents.
Summary
In summary, alkanes, alkenes and alkynes are three types of hydrocarbons that differ in their carbon-carbon bonding and their general formulas. Alkanes have only single bonds and are saturated, alkenes have at least one double bond and are unsaturated, and alkynes have at least one triple bond and are unsaturated. The degree of unsaturation affects the reactivity and stability of the hydrocarbons, as well as their physical properties, such as boiling point, melting point, density and solubility.
References
[13.1: Alkenes- Structures and Names - Chemistry LibreTexts]
[Chapter 8 - Alkenes, Alkynes and Aromatic Compounds]
[Hydrocarbons â organic chemistry worksheets 14â16]
[Alkenes â organic chemistry worksheets 14â16 - RSC Education]
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