This is the correct chemical term for compounds known as paraffins. They are considered the simplest organic compounds and are a family of chain hydrocarbons having the general formula C2H2n+2. All of the bonds are single bonds (-C-H-, and -C-C-). The chains can be straight or branched. The smaller members (less than 4 carbons) are gases, while larger ones (five to seventeen carbons) are liquids. Beyond seventeen carbons the alkanes are waxy solids. Structure: The simplest saturated acyclic hydrocarbon is methane, CH4.
The other hydrocarbons belonging to this class can be taken as descendents of methane, resulted from the substitution of one or more atoms of hydrogen with hydrocarbon radicals. Therefore, if one hydrogen atom from methane is replaced by a methyl radical, -CH3, the hydrocarbon (superior to methane) will have the composition C2H6, named ethane. The alkanes can be: with a continuous chain, if all the carbon atoms are bound with at most two other carbon atoms, and with a branched chain, if one or more carbon atoms from the molecule are bound with at least two other carbon atoms.
Such an isomerism of chain is possible starting with butane. The number of isomers of saturated acyclic hydrocarbons is very big; he grows with each number of carbon atoms from the molecule. For example, the carbide C10H22 has 75 isomers; the hydrocarbon C20H42 can have approximately 366. 319 isomers. Nomenclature The first four alkanes with a unbranched chain, meaning normal (n-alkanes) are called: methane, ethane, propane, butane. The superior alkanes names are compound with a numeric prefix (which shows the number of carbon atoms present in the molecule) and the ending “-ane”.
For example: pentane, hexane, heptane, and octane. For the hydrocarbons with two groups of methyl (-CH3) at the end of a linear chain, you use the prefix “iso-” at the name of the hydrocarbon, and for those containing three -CH3 groups at the end of the chain, you use the prefix “neo-” at the name of the hydrocarbon. Mono valence radicals derived from normal alkanes by removing one hydrogen atom from a terminal carbon atom are called normal alkyls. Their name is formed by the replacement of the ending “-ane” from the hydrocarbon’s name with the ending “-yl”.
For example, CH3 is called methyl; CH3-CH2 is called ethyl and CH3-CH2-CH2 is called propyl. The branched chain alkanes use the name of the longest sidewise chain as a prefix in their denomination. The position of the sidewise chain is indicated by a number. For this, the longest chain is numbered from left to right or vice verse, in the direction in which the sidewise chain will have the position with the smallest number. The presence of more identical sidewise chains is indicated by the prefixes: di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.
Bivalence and tri-valence radicals derived from alkyls by removing one or two hydrogen atoms from the mono-valence carbon atom, have their names formed by adding the ending “-iden” or “-idin”. For example, CH3-CH= is called ethyliden. The radical CH2= is named methylene, CH is name ethylidin. General formula: In a alkane molecule at n carbon atoms you fin 2n+2 hydrogen atoms. Therefore the general formula of the alkanes is CnH2n+2. Relying on the general formula you can obtain the molecular formula of each and every alkane if the number of carbon atoms is known.
For example, for a molecule which contains 2 carbon atoms (n=2), the number of hydrogen atoms is 2*2+2=6 so the hydrocarbon’s molecular formula is C2H6 (ethane). Alkenes Acyclic hydrocarbons containing a double bound within two carbon atoms in the molecule are named alkenes. They are also called olefins, because of the fact that the chlorate and bromate derivates are oleaginous liquids. Structure: The simplest alkene is ethylene (C2H4), which has the International Union of Pure and Applied Chemistry name ethene. This substance contains two carbon atoms connected through a double bound.
Superior alkenes can be considered as the result of the substitution of a hydrogen atom with alkyl radicals, same as the case of saturated acyclic hydrocarbons. Hence, the superior homologue of ethylene is propene, CH2=CH-CH3. The homologue of propene is buthene, C4H8. After the position of the double bound in the molecule at buthene, you can distinguish three position isomers. Because of the double bound in the molecule, alkenes have two hydrogen atoms less then other saturated hydrocarbons; therefore, the general formula for these hydrocarbons is CnH2n. You can observe the homologous series of the cycloalkanes.
They are, consequently, raw formula isomers. Physical properties: C2-C4 alkenes are gases, C5-C18 are liquids and the ones with more than 18 carbon atoms are solids. The boiling points are a little bit lower than the corresponding alkanes. Their density is higher than the corresponding alkenes, but all alkenes have the density smaller than 1. Alkenes are insoluble in water, but mixable with organic solvents. Alkynes The acyclic hydrocarbons with one triple bound in the molecule are called alkynes. The still customary denomination of acetylene comes from the series first hydrocarbon, acetylene.
Structure: Acetylene has the composition C2H2: it contains two carbon atoms connected through a triple bound: CHCH. The C-H bounds of acetylene are formed by the interpenetration of an orbital sp from the carbon atom with an orbital s from the hydrogen atom; the triple bound is made of one sigma bound (formed by the interpenetration of two orbital sp’s from each carbon atom) and two pi bounds (result of the interpenetration of two pairs of orbital p’s). Same as for ethylene, for detaching the pi bounds a smaller quantity of energy is necessary unlike for detaching the sigma bounds.
The isomers for compounds with triple bound are unknown, because all the atoms are positioned collinear. Acetylene derivates (obtained through the substitution of one or both hydrogen atoms with radical alkyls) can be considered the homologues of acetylene. Therefore, by substituting one hydrogen atom with one methyl radical you obtain propine C3H4: CHC-CH3. General formula:Unsaturated hydrocarbons with a triple bound in the molecule, containing 2 atoms less than the corresponding saturated hydrocarbons, have the general formula CnH2n-2.
Physical properties:At usual temperature, the C2-C4 alkynes are gases, C4-C14 are liquids, and the superior ones are solids. Their boiling points are similar, yet a bit higher than the alknes and alkenes points. Their solubility is higher than their corresponding saturated hydrocarbons. Chemical properties:The chemical behavior of the alkynes is characterized by their capacity of giving addition reactions, polymerization reactions and metallic derivates forming reactions. A great part of reactions are known as belonging to acetylene.