Organic substances formed by the two elements of carbon and hydrogen are called hydrocarbons. According to the bonding mode between carbon atoms in the molecule, they can be divided into saturated hydrocarbons and unsaturated hydrocarbons. Among them, alkanes are saturated hydrocarbons, while alkenes and alkynes are unsaturated hydrocarbons.
What is the Definition and Properties of Alkanes?
Open-chain saturated hydrocarbons are called alkanes. Alkanes refer to compounds in which the carbon atoms in the molecule are connected by single bonds, and the remaining valence bonds are all bonded to hydrogen. The simplest alkane is a compound containing one carbon atom, called methane, with a molecular formula of CH4. The one containing two carbon atoms isethane, with a molecular formula of C2H6. As the number of carbon atoms gradually increases, a series of compounds can be obtained. Starting from methane, for every additional carbon atom, two hydrogen atoms are added accordingly. Therefore, the composition of this series of compounds can be expressed by a formula such as CnH2n+2, which is called the general formula of alkanes.
Generally speaking, the physical constants of each substance in the same series change with the increase of relative molecular mass. For example, the boiling point and relative density of alkanes increase with the increase of relative molecular mass. At room temperature, the first four compounds in the alkane series are gases, starting from pentane, they are liquids, and above octadecane are solids. The boiling point and melting point of a substance increase with the increase of relative molecular mass because it is related to the intermolecular force. The relative density of alkanes also increases with the increase of relative molecular mass.
Alkanes contain only C-C bonds and C-H bonds in their molecules. Both bonds are very strong, and the electronegativity of carbon and hydrogen is very small, so the polarity of C-H bonds is very small. Therefore, compared with other organic substances, alkanes have considerable chemical stability to ionic reagents. Under normal circumstances, alkanes do not react with most reagents, such as strong acids, strong bases, strong oxidants, etc. However, under certain conditions, such as at high temperatures or in the presence of catalysts, alkanes can also react with some reagents.
What is the Definition and Properties of Alkenes?
Alkenes are a type of unsaturated hydrocarbon. In the homologous series of alkenes, the simplest one is ethylene. As can be seen from the above example, alkenes containing one C=C have two fewer hydrogen atoms than the corresponding alkanes, so their general formula is CnH2n. Carbon-carbon double bonds are the functional groups of alkenes.
The physical properties of alkenes are very similar to those of alkanes. Alkenes with less than four carbon atoms are gases at room temperature, and higher homologous series are solids. Alkenes are lighter than water, and asymmetric alkenes have weak polarity. Alkenes are insoluble in water, but easily soluble in non-polar or weakly polar organic solvents such as benzene, ether and chloroform. For the cis-trans isomers of alkenes, such as 2-butene, the dipole moments of the bonds of the trans isomers cancel each other out, and the dipole moment of the molecule is zero, while the cis isomers are dipole molecules with weak polarity, so the boiling point of the cis isomer is generally slightly higher than that of the trans isomers. On the contrary, the molecules of the trans isomers can be arranged more tightly in the lattice, so the melting point is higher than that of the cis isomers.
In the carbon-carbon double bond, one is a σ bond and the other is a π bond. The π bond is formed by the overlap of p orbitals on the side, and its overlap is worse than that of the σ bond. In addition, the π bond electron cloud is distributed on the upper and lower sides of the carbon-carbon bond axis. Compared with the σ bond, it is exposed to the outside of the molecule and is less affected by the nucleus. It is easy to give its pair of electrons to the electrophilic reagent. Therefore, the carbon-carbon double bond is the functional group of the alkene, and its main reaction is addition reaction, thereby generating saturated compounds.
What is the Definition and Properties of Alkynes?
Hydrocarbons containing a carbon-carbon triple bond (C≡C triple bond) in their molecules are called alkynes. Alkynes are also unsaturated hydrocarbons. They have two fewer hydrogen atoms than the corresponding alkenes, so their general formula is C2H2n-2. The simplest and most important one in the alkyne series is acetylene. The carbon-carbon triple bond is the functional group of alkynes.
The boiling point and relative density of alkynes are slightly higher than those of the corresponding alkenes. Alkynes with less than four carbon atoms are gases at room temperature and pressure. Alkynes are lighter than water, have weak polarity, are insoluble in water, but are easily soluble in organic solvents such as petroleum ether, benzene, ether and acetone. At 15°C under normal pressure, 1 volume of acetone can dissolve 25 volumes of acetylene. Pure acetylene is a colorless and odorless gas. Acetylene generated by the hydrolysis of calcium carbide has a special odor because it is mixed with a small amount of sulfur-containing and phosphorus-containing compounds. Acetylene emits a bright flame when it burns. The flame temperature of acetylene burning in oxygen can reach up to 3500℃, so it can be used to melt and weld metals.
Alkynes contain carbon-carbon unsaturated bonds and can undergo similar reactions to alkenes. For example, hydrogen, halogens, hydrogen halides and water can all be added to alkynes, and the C≡C triple bond can also be oxidized and broken to form carboxylic acids. Since alkynes contain two bonds, the addition can be carried out step by step. Under appropriate conditions, the product of the addition with one molecule of reagent can be obtained. The product of the reaction with two molecules of reagent is an alkane or its derivative. The addition of halogens, hydrogen halides and water to alkynes is also electrophilic addition.
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