Alkanas: Nomenclature of Hydrocarbon Compounds

Nomenclature of Hydrocarbon Compounds : Alkanes a. General Formula of Alkane Alkanes are saturated aliphatic hydrocarbon compounds, namely hydrocarbons with open chains and all carbon bonds are singlebonds. Alkane compounds have the formula (James E. Brady): CnH2n + 2 The names of the ten alkanes with the number of carbon atoms 1 to 10 are in table below. This is the basis of the names throughout organic compounds. b. Alkyl group The alkyl group is an alkane that has lost an H atom. This alkyl can be written using the formula: CnH2n + 1 By replacing one H atom, the name will also be changes from methane to methyl. Here are some alkyl groups commonly used. The methyl group and ethyl group are only one of a kind, namely: There are two types of propyl groups, namely: Meanwhile, there are four types of butyl groups, namely: c. Nomenclature of Alkanes In naming this alkane it will be very difficult if you only use the usual alkane nomenclature (methane to decane, for C1 – C10). This is due to the presence of isomers in alkanes, so special names are needed. For example, the normal prefix is used for straight chains, while the iso prefix is for isomers that have one CH3 branch attached to the number two carbon atom. Even though it is very difficult for us to give names to carbon chains that have many isomers. Therefore, the international chemist association at a meeting in Geneva in 1892 had formulated rules for naming chemical compounds. The nomenclature they formulated is known as the IUPAC (International Union of Pure and Applied Chemistry) nomenclature. Names derived with this rule are called systematic names or IUPAC names, while names that were commonly used before the IUPAC nomenclature are still used and are called ordinary names or trivial names The IUPAC rules for naming branched alkanes are as follows. 1) The name of a branched alkane consists of two parts, namely: a) The first part, on the front, is the name of the branch (branch branch). b) The second part, on the back, is the name of the parent chain Example: 2 methylbutane – Main Branch 2) The parent chain is the longest chain in the molecule. If there are two or more longest chains, the one with the most branches must be chosen. The parent is given the name of the alkane, depending on the chain length. Example: 3) The branch is named alkyl, that is, the name of the appropriate alkane by changing the ending ana to il. The alkyl group has the general formula Cn H2n + 1 and is represented by the symbol R (see about alkyls). 4) The position of the branch is stated with a numeric prefix. For that main chain needs to be numbered. Numbering starts from one end of the main chain so that the position of the branch gets the smallest number. Example: 5) If there are two or more of the same branches, this is indicated by the prefix di, tri, tetra, penta, and so on in the name of the branch. 6) The different branches are arranged in alphabetical order of name that branch. For example : • Ethyl is written before methyl. • Isopropyl is written before methyl. Based on these rules, the naming of alkanes can be done in the following steps. 1) Choose the parent chain, which is the longest chain that has the most branches. 2) Give numbering starting from one end, so that the branch gets the smallest number. 3) Write the name starting with the name of the branch arranged alphabetically, then ending with the name of the parent chain. Branch positions are indicated by prefix numbers. Between numbers and numbers are separated by a comma (,), whilebetween numbers and letters are separated by a pause (–). The following is an example of naming alkanes. d. Isomerization of Alkanes As we have learned before, the term isomers are known in hydrocarbon compounds. Isomers that occur in alkanes are skeletal isomers. For example C5 H12 has isomers: That is, a compound with the molecular formula C5 H12 has 3 isomers. What about the other molecular formulas? Make isomers from C7 H16 and C8 H18! e. Alkane properties 1) All hydrocarbons are nonpolar compounds, so they are insoluble in water. If a hydrocarbon is mixed with water, then the hydrocarbon layer is always on top because the density is less than 1. Good solvents for hydrocarbons are nonpolar solvents, such as CCl4 or ether. 2) The more C atoms, the higher the boiling point. For hydrocarbons with isomers (same number of C atoms), the boiling point is higher when the C chain is longer (less branched). 3) At ordinary temperature and pressure, the first four alkanes (CH4 to C4 H10) are gases. Pentane (C5H12 ) to heptadecane (C17H36) are liquid, while octadecane (C18H38) and so on are solid. 4) If it is reacted with halogen elements (F2 , Cl2 , Br2 , and I2 ), then the H atoms in alkanes are easily replaced (exchanged) by halogen atoms. CH4 + Cl2 CH3 Cl + Cl2 CH2 Cl2 + Cl2 CHCl3 + Cl2 CH3 Cl + HCl methylchloride (chloromethane) CH2 Cl2 + HCl dichloromethane CHCl3 + HCl chloroform (trichloromethane) CCl4 + HCl carbon tetrachloride 5) Alkanes can undergo oxidation with oxygen gas, and this combustion reaction always produces energy. That is why alkanes are used as fuel. On average, the oxidation of 1 gram of alkanes produces 50,000 joules of energy. Complete combustion reaction: CH4 + 2 O2 CO2 + 2 H2 O + energy Incomplete combustion reaction: CH4 + O2 CO + 2 H2 O + energy