Branched-chain hydrocarbons naming

Paraffins are straight or branched chain hydrocarbons having the chemical formula C ii2 +2- The name of each member ends with ane examples are propane, isopentane, and normal heptane (Figure 2-1). 

The names of branched-chain hydrocarbons and hydrocarbon derivatives are based on the name of the longest continuous carbon chain in the molecule (which may not be shown in a horizontal line). 

HC Unibon [Hydrocracking] A version of the hydrocracking process for simultaneously hydrogenating and cracking various liquid petroleum fractions to form branched-chain hydrocarbon mixtures of lower molecular weight. The catalyst is dual-functional, typically silica and alumina with a base metal, in a fixed bed. Developed by UOP. By 1988,46 licenses had been granted. Currently offered under the name Unicracking. 

The windows to the channels thus form a three-dimensional sieve with mesh widths between about 300 and 1000 pm, thus the well-known name molecular sieve for these crystalline aluminosilicates. Zeolites thus have large internal surface areas and high sorption capacities for molecules small enough to pass through the window into the cavities. They can be used to separate mixtures such as straight-chain and branched-chain hydrocarbons. 

The continuous chain hydrocarbons are known as normal hydrocarbons, and the prefix n- is usually attached to the name. The branched-chain hydrocarbons may have the prefix iso- attached to the name. Usually the prefix iso- is reserved for substances with two methyl groups attached to carbon atoms at the end of an otherwise straight chain. In a straight chain, each carbon atom is connected to no more than two other carbon atoms. The prefix neo- denotes three methyl groups on a carbon atom at the end of a chain. For example, the isomers of pentane are illustrated below. 

Branched-chain substituent groups are given appropriate names by a simple extension of the system used for branched-chain hydrocarbons. The longest chain of the substituent is numbered starting with the carbon attached directly to the parent hydrocarbon chain. Parentheses separate the numbering of the substituent from the main hydrocarbon chain. 

The possibility of having branched-chain hydrocarbons that are structural isomers of the continuous-chain hydrocarbons begins with butane (n = 4). The HJPAC rules for the systematic naming of these hydrocarbons follow. 

So far, we have just looked at hydrocarbons that are straight chains. There are many compounds where central carbon atoms are bonded to three or four carbon atoms or other non-hydrogen atoms. For branched-chain hydrocarbons, there are special procedures for naming compounds. To start with, we will only consider saturated hydrocarbons. Once you understand some of the basic rules of nomenclature, we can begin to look at unsaturated hydrocarbons and atoms other than carbon and hydrogen. 

The alkanes, also known as paraffins or saturated hydrocarbons, are straight- or branched-chain hydrocarbons with only single covalent bonds between the carbon atoms. We will study the alkanes in some detail because many other classes of organic compounds are derivatives of these substances. Be sure to learn the names of the first 10 members of the alkane series, because they are the basis for naming other classes of compounds. 

The following rules enable us to name branched-chain hydrocarbons unambiguously as long as we apply the rules in sequence. 

Unsaturated branched acyclic hydrocarbons are named as derivatives of the chain that contains the maximum number of double and/or triple bonds. When a choice exists, priority goes in sequence to (1) the chain with the greatest number of carbon atoms and (2) the chain containing the maximum number of double bonds. 

Alkanes are a class of saturated hydrocarbons with the general formula C H2n. -2- They contain no functional groups, are relatively inert, and can be either straight-chain (normal) or branched. Alkanes are named by a series of IUPAC rules of nomenclature. Compounds that have the same chemical formula but different structures are called isomers. More specifically, compounds such as butane and isobutane, which differ in their connections between atoms, are called constitutional isomers. 

The chain and branched chain saturated hydrocarbons make up a family called the alkanes. Some saturated hydrocarbons with five carbon atoms are shown in Figure 18-11. The first example, containing no branches, is called normal-pentane or, briefly, n-pentane. The second example has a single branch at the end of the chain. Such a structural type is commonly identified by the prefix iso- . Hence this isomer is called /50-pentane. The third example in Figure 18-11 also contains five carbon atoms but it contains the distinctive feature of a cyclic carbon structure. Such a compound is identified by the prefix cyclo in its name—in the case shown, cyclopentane. 

The use of the old terminology n- (normal) for unbranched hydrocarbon chains, with i- (iso), s-(secondary), t- (tertiary) for branched chains is still quite common with small molecules, and can be acceptable in lUPAC names. 

Selection of the principal chain in unsaturated branched acyclic hydrocarbons. A principal chain must be chosen upon which to base the name of branched unsaturated acyclic hydrocarbons. The general criteria listed in Table 4.10 are applied. Those that are specifically relevant to polyenes and polyynes are criterion (b), which is the maximum number of double and triple bonds considered together criterion (c), which is the maximum length criteria (d), (f) and (g) criterion (h), which is the maximum number of substituents cited as prefixes and criteria (i) and (j). 

Other types of alkanes include closed circles and branched chains, but we discuss the straight-chain alkanes here because they make clecir the basic strategy for naming hydrocarbons. From the standpoint of nciming, the hydrogen atoms in a hydrocarbon are more or less filler atoms. Alkanes names cire based on the largest number of consecutively bonded carbon atoms, so the ncime of a hydrocarbon tells you about that molecule s structure. 

When every carbon atom in a hydrocarbon except the two terminal ones is bonded to only two other carbon atoms, the molecule is called a straight-chain hydrocarbon. (Do not take this name literally, for, as the w-pentane structures in Figure 12.1 show, this is a straight-chain hydrocarbon despite the zigzag nature of the drawings representing it.) When at least one carbon atom in a hydrocarbon is bonded to either three or four carbon atoms, the molecule is a branched hydrocarbon. Both wo-pentane and /w-pentane are branched hydrocarbons. 

A saturated branched acyclic hydrocarbon is named by numbering the longest chain from one end to the other, and the positions of the side chains are indicated by the lowest possible numbers. The numbers precede the group, and are separated from them by a hyphen. 

Names of branched-chain unsaturated aliphatic hydrocarbons (IUPAC rules) are formed from the name of the longest fundamental chain present in a formula and the names of the side chains with proper position designations. The resulting names... 

To discern how many carbon atoms are in a straight chain or branch of a carbon compound, prefixes are used. If all the carbon-carbon bonds are single bonds, the hydrocarbon name ends in -ane. These molecules are called alkanes. If there is a double carbon-carbon bond, the name ends vn-ene. These molecules are called alkenes. If there is a triple carbon-carbon bond, the name ends in -yne. These molecules are called alkynes. 

The correct numbering shows us that the branch comes from the third carbon atom in the straight chain. Therefore, the hydrocarbon name begins with the number 3. There is one carbon atom in the branch, so the branch name begins with meth- and ends in -y/—methyl. The straight chain has six carbon atoms (hex-) with only single bonds (-ane), so it is named hexane. Put it all together and we have 3-methylhexane. 

For example, butane has a boiling point of -0.5°C, while 2-methylpropane s boiling point is -11.7°C. Only hydrocarbon chains with at least four carbon atoms are capable of forming a branch. Because there must be a way to distinguish isomers, the nomenclature must take these into account. The following are general rules for naming branched-chain alkanes ... 

These rules work for hydrocarbon frameworks that are chains or rings, but many skeletons are branched. We can name these by treating the branch as though it were a functional group ... 

The names of branched-chain alkanes (and most other aliphatic compounds) have the same general format, as shown in Figure 13.14. This format will become clearer as you learn and practise the rules for naming hydrocarbons. To start, read the steps on the next page to see how 2-methylpentane gets its name. 

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