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Naming Hydrocarbons

The aim of organic chemistry nomenclature is to give every compound a unique identity which will convey without any ambiguity the exact structure of the compound.

The system bases on the names of all compounds on accepted norms of hydrocarbons and unsubstituted heterocycles or the parent compound. The appropriate replacement of hydrogen atoms in parent compounds by other atoms or groups is indicated by numbered prefixes, suffixes, and other methods.

Many of the names first given to compounds frequently before their constitution was known are firmly embedded in the system itself and some others form the names of the parent hydrocarbons while others give the class to retain these as convenient short names for compounds of complex constituents.

There is usually no great difficulty in devising an unambiguous name for most organic compounds but difficulties arise when it becomes desirable to select unique names which are most for retrieving the compounds specific information.


Rules for Naming Hydrocarbons

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The entire gamut of organic nomenclature deals with both closed chain and open chain forms and there are specific rules which are universal and easy to comprehend by everyone who deals or uses these compounds in the right perspective.

 Open chain aliphatic hydrocarbons

Saturated straight chain hydrocarbons having up to four carbon atoms have special names of historic significance like methane, ethane, propane, and butane. Anything higher than these is taken from Greek numerals indicating the number of carbon atoms present in the molecule.

The names of saturated hydrocarbons have the suffix of –ane and they are given the general class name of the alkane. 

These names are used only for the straight chain hydrocarbons and additional root names are used for the unsubstituted branched isomers.

Isobutene – $(CH_{3})_{2}CH. CH_{3}$

Isopentane -- $(CH_{3})_{2}CH.CH_{2}.CH_{3}$

Neopentane -- $(CH_{3})_{4} C$

Isohexane -- $(CH_{3})_{2}CH. CH_{2}.CH_{2}.CH_{3}$

All other saturated open chain hydrocarbons having branched chains are regarded as substitution derivatives of the hydrocarbon corresponding to the longest chain in the given molecule, while the substituent being residues or radicals remaining when a terminal hydrogen atom is removed from a saturated hydrocarbon.

These radical names are formed from the root name of the parent hydrocarbon with the ending –yl and thus –CH3 becomes methyl etc.

$(CH_{3})_{2}CH-$ is isopropyl
$(CH_{3})_{2}CH. CH_{2}-$ is isobutyl
$(CH_{3})_{3} C$ – is tert – butyl

The location of individual carbon atoms in straight chain hydrocarbons is achieved by the serial allocation of Arabic numerals and the direction of numbering is so chosen that the lowest numbers possible are given to carbons atoms bearing side chains or the lower number being given the carbon atom having the least complex side chain.

The substituents are arranged in alphabetical order and ignore plurality indicators before the root name.

Example: The compound $C_{9}H_{2}0$ can be represented as 

The name for this structural form of the compound can be 4-ethyl 3-methyl hexane and not named as 3-methyl 4-ethyl hexane. A branch chain is similarly numbered from the point of attachment to the main chain where the numbers are primed.

Hydrocarbons having double bond or olefins have the same root name as the corresponding saturated hydrocarbons but have the ending of –ene and they are generally classed as an alkene.

In case there are more than one double bonds then these of double bonds indicated by ending with a diene, triene etc. 

The positions of these double bonds are indicated by quoting the number of the lower carbon affected and hence $CH_{2}: CH.CH_{2}.CH_{3}$ named as but – 1 – ene or 1 – butene or even butene – 1

In case there is more than one double bond like in $CH_{2}: CH.CH: CH.CH_{3}$ is written as penta 1, 3 – diene or even as 1, 3 penta diene.

Radicals containing double bonds usually consider the ending –enyl and are again formed by removing a terminal hydrogen so that the point of attachment of the radical is always the position 1.

Molecules which are unsaturated and branched acyclic hydrocarbons, they are named as substitution derivatives of the straight chain molecule which has the maximum number of double and triple bonds and also the chain containing the greatest number of carbon atoms.

Hydrocarbons containing a triple bond take the root name of the corresponding saturated hydrocarbon with the ending of – yne.

Hence, methyl acetylene can be written more appropriately as prop- 1-yne. The presence of both double bonds and triple bonds in a molecule requires the ending –en-yne where the numbers locating the particular unsaturation are used before appropriate symbols.

The numbers used are the lowest possible for both types of unsaturation and where there is a choice of equally low numbers, the double bond is denoted by a lower number than the one locating the triple bond.

General principle for naming derivatives

The name of any compound other than a hydrocarbon or heterocycle consists of at least two parts, one indicating the parent system on which the compound is based and the attached groups or the so-called functional groups and non-functional groups or atoms.
These are mainly halogens, nitro, nitroso, and azoxy as given out by IUPAC rules.

In-addition the positions of all attached atoms or groups and repetition of substituents are indicated.

In systematic organic nomenclature, the main functional group of a compound is expressed as a suffix.

Earlier the suffix was used separately as in case of R – CO – R (where R is the alkyl group) and these are nomenlcated like ethyl methyl ketone but now its more systematic and is named as butane â€“ 2 – one. The earlier forms are now avoided to streamline the nomenclature.

All substituents other than the principal function are expressed as prefixes as they arranged in alphabetical order and ignore the presence of more than one of same type substituents.

In case there is a presence of more than one functional group the principal function is regarded as that occurring first.

The order is as follows:

Onium compound, -- acid (carboxylic, arsonic, sulphonic, stibonic), -- acid halide, -- amide, -- imide, -- amidine, -- nitrile, -- isocynide, ketone, -- alcohol, -- phenol, -- thiol, -- amine, -- ether, -- sulphide, -- sulphoxide

IUPAC Nomenclature of Hydrocarbons

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Many years ago it was clear that in chemistry and especially in Organic chemistry, the trivial names or common names cannot reply upon.
The rules of the system result in a unique name for each compound. Knowing the rules and observing a structure can help us write the systematic name and by seeing the systematic name we can redraw the structure.

Finally the International Union of Pure and applied chemistry came up with a system where each structure can be written using the rules laid out and anyone seeing these formulas can redraw the structure. However, some of the widely used common names were retained in order to make it popular and user-friendly.
  • The general name for any acyclic saturated hydrocarbon is alkanes and the ending is –ane. This is used for all saturated hydrocarbons and for another type of endings will be used for other types of functional groups.
  • The alkanes without branch are named according to the number of carbon atoms.
  • Alkanes with branches, the root name is given to the longest continuous chain of carbon atoms.
  • The functional groups attached to the main chain are called substituents. 
  • The main chain is numbered in such a way that the first substituent encounterd along the chain recieves the lowest count of number.
  • Each substituent is then located by its name and the number of the carbon atom to which it is attached.
  • In case there are two or more identical groups are attached to the main chain we give prefixes as di, tri or tetra
  • Every substituent must be named and numbered even if two identical substituents are attached to same carbon atom of the main chain.
  • In case we get to see more than one type of functional groups then we need to arrange the substituents alphabetically except that prefixes such as di, tri are not considered
  • Punctuation of numbers while writing IUPAC names is must
  • Alkyl substituents are named by changing the –ane ending of alkanes to – yl
  • Alkene nomenclature is based on general principle of IUPAC system with a suffix of – ene
If there are more than double bonds in a molecule, the location is indicated by a number and the number assigned is that of lower numbered carbon involved with double bond
  • In case of cyclic compounds the double bond is given the lowest number count
  • In case a molecule having a double bond as well as alcohol suffix of – ol then the suffix is given out as enol
  • In case of more than one double bond then the di, tri etc is used with a suffix of –ene
  • The rules of alkynes are almost identical to that of alkenes except the use of suffix of –yne

Naming Hydrocarbons with Functional Groups

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The classification of organic compounds is based mainly on functional groups which determine the chemistry. The names of organic compounds are derived from the functional group and the main carbon chain.

The structures are then drawn from the names of functional groups in context with Kekule, condensed or skeletal structures. Functional group is made up of an atom or atoms with characteristic chemical properties.
  • Hydrocarbons with only H and C present.
Alkane, alkene or alkyne. Alkanes are saturated as they have the highest number of hydrogen atoms as compared to alkenes and alkynes.
  • Carbon bonded to an electronegative atom
Single bond groups are as follows:

R – X (halide)
R – OH (alcohol)
R – O – R (ether)
R – NO2 (nitro)
R – SH (thiol)
R – S – R (thio ether)
R – NH2 (primary amine)
R – NHR (secondary amine)
R – NR2 (tertiary amine)
Double bonds to oxygen compounds are as follows:
R – CHO (Aldehyde)
R – COR (ketone)
R – COX (acid halide)
R – COOH (carboxylic)
R – COOR (ester)
R – CONH2 (amide)
Triple bond to nitrogen
R – CN (nitrile)
  • Identifying substituents on main chain and providing specific numbers
  • Two substituents on same carbon are given same number and the substituents name and position is the prefix 
Carboxylic Compounds
Minor functional group prefix are as follows:
  • Chloride as chloro 
  • Bromide as bromo
  • Iodide as Iodo
  • Alcohol as hydroxy 
  • Ether as alkoxy 
  • Aldehyde as formyl 
  • Ketone as Oxo 
  • Nitro as nitro 
  • Amine as amino 
  • Nitrile as Cyano
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