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Naming Alkyl Halides

Carbon an element that is placed in 14th group of periodic table play an important role in our everyday life. It has ability to form a variety of compounds by bonding with other elements. These carbon compounds are directly or indirectly related to living organisms.

The catenation property of carbon atom makes it unique and different from other elements. The branch of chemistry which mainly deals with carbon compounds is called as organic chemistry.  Carbon atom exhibits tetravalency and can form four covalent bond with same or other elements. 

The bonding with hydrogen results the formation of most common carbon compounds called as hydrocarbon. Hydrocarbons contain single, double or triple bonds between carbon chains of molecule. On the basis of bonds between carbon atoms, hydrocarbons can be classified as alkane, alkene and alkynes. 

As the number of organic compounds increases, it became necessary of naming these compounds to identify them from other compounds. The naming of organic compounds started from trivial names which were based on origin. 

These are nonsystematic names for a chemical compounds which do not help to recognize the structure of respective compounds. So we can say these are not a formal name but are usually common names.

 Name   Formula 
 Methane   $CH_{4}$
 Butane  $C_{4}H_{10}$ 
 Acetone  $CH_{3}COCH_{3}$  
 Toluene  $CH_{3}C_{6}H_{5}$
 Acetylene   $C_{2}H_{2}$
 Ethyl Alcohol   $C_{2}H_{5}OH$
 Acetic acid  $CH_{3}COOH$
 Oxalic acid  HOOC-COOH 


Alkyl Halide Nomenclature

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Since the trivial names or common names could not provide all the information about molecule and it was difficult to give trivial name for all the known compounds, so a systematic naming was required which can provide name to any of the chemical compound. The International Union of Pure and Applied Chemistry suggested some rules for naming of organic and inorganic compounds. 

If there is not functional group in the molecule, the compound will be composed of only carbon and hydrogen atoms bonded together by sigma bonds. Such compounds are called as alkanes. Some of the common functional groups with their suffix and prefix are given below.

   Functional group   Structure   Suffix 
 1.  Alkane  -C-C-  -ane
 2.  Alkene  >C=C<  -ene
 3.  Alkyne  -C≡C-  -yne
 4.  Aldehyde  -CHO  -al
 5.  Ketone  >C=O  -one
 6.  Amine  -NH2  -amine 
 7.  Ether  -O-  Alkoxyalkane 
 8.  Ester  -COO-  -oate
 9.  Nitrile  -C≡N  -nitrile
 10.  Carboxylic acid  -COOH  -oic acid
 11.  Halide  -X  -
 12.  Alcohol  -OH  -ol
 13.  Acid halide  -COX  -oyl halide
 14.  Amide  $-CONH_{2}$   -amide
 15.  Nitro  $-NO_{2}$  -

Look at the list of functional groups. Here some of the groups do not have any prefix like alkane, alkene, alkyne etc. whereas some of them do not have any suffix like nitro, halo. The IUPAC naming is based on three important parts of name of any organic compound;
Prefix  +  Root word / base word + Suffix = IUPAC name of organic compound 

Some of the compounds may not have prefix but root word and suffix are always there. Let’s discuss these terms one by one then we will come to IUPAC rules as by knowing these rules and given a structural formula, we can write a unique name for every distinct compound.

Root word: It indicates the number of carbon atoms in the longest chain of given structure. Later we have to identify and locate any functional groups present in the compound as presence of hydrogen can be assumed from the tetravalency of carbon. Therefore presence of hydrogen need not be specified in most of cases. 

 Number of Carbon atoms   Root word 
 1.  Meth-
 2.  Eth-
 3.  Prop-
 4.  But-
 5.  Pent-
 6.  Hex-
 7.  Hept-
 8.  Oct-
 9.  Non-
 10.  Dec-
 11.  Undec-
 12.  Dodec-

  • Prefix- It indicates the presence of any substituent bonded on parent chain of compound. The location of prefix is always represented with number before their name, separated by hyphen. Some of the functional groups like halo group, nitro group are written as prefix. 
  • Suffix- A suffix represents the functional group in the compound. 

The IUPAC sets some logical rules which are used by organic chemists to circumvent problems caused by arbitrary nomenclature. It provides a systematic nomenclature system which applicable on almost all organic compounds. In IUPAC naming system, the base part of the name reflects the number of carbons that is called as parent chain whereas suffix of the name reflects the functional group present on the parent chain. Substituents are other groups which are attached to the parent chain of compound. Let’s discuss the IUPAC rules with simplest organic compounds, Alkanes. Alkanes are saturated hydrocarbons with single bonded carbon atoms in the parent chain. The suffix used for alkanes is –ane and substituent must be written with their location as prefix. The names of substituents must be written by changing the suffix -ane to –yl such as methyl-, ethyl- and so on. Some other substituents are given below.

Butyl Group

To write the IUPAC name of alkanes, first identify the longest chain of carbon atoms in the molecule that is called as parent chain. The number of carbon atoms in parent chain will determine the root word of the molecule. Since all the carbon atoms are bonded with single bonds so the suffix must be –ane. Now next step is identification of substituents which are groups appending from the parent chain.

The numbering of carbons of the parent chain must be in this way that gives lowest number to substituents appending from the parent chain. If there are two side chains in equivalent positions, assign the lowest number to that sided chain which will come first in the name. In case of two same substituents, the location of each point with the number of times the substituent group occurs must be indicated with prefix di, tri, tetra, etc. such as 2,2-dimethyl. 

Another condition is presence of two or more different substituents. In this case we have to follow the alphabetical order using the base name. Remember only iso prefix is used in alphabetical order whereas prefixes sec- and tert- are not used in determining alphabetical order. If more than one ways to select the numbering of parent chain, we should follow 

  • the chain with greatest number of side chains,
  • the chain with lowest- numbers substituents,
  • the chain with greatest number of carbon atoms in the smaller substituent,
  • the chain with the least branched substituents.

Put commas between numbers and dashes are used between letters and numbers. The complete IUPAC name will be without any spaces in the name.

Alkyl halides are organic compounds with any halogen group ( -F, -Cl, Br and –I) bonded on parent chain. The systamatic name of alkyl halide is haloalkane as halo group is always come as prefix with parent carbon chain. For the naming of alkyl halides, we have to follow same rules of IUPAC as we have discussed for alkanes. Here the substitutent will be halo group with may or maynot an alkyl group. Let’s take an example.

Bromo Alkane

Here the given image of alkyl halide contains total 8 carbon atoms so we have to check the longest chain with lowest number to substituent that is –Br group. There are 7 carbon atoms in longet chain and we can number the chain from any side as the side chain will get location 4 in both cases. Hence the root word will be hept + -ane = haptane.
Primary Bromo Alkane

Now here the side chain consists of $–CH_{2}Br$. The prefix for $–CH_{3}$ is methyl and here one of the H is replaced by Br group so the name of side chain will be bromomethyl at position 4.
Alkyl Bromide

Hence the IUPAC name of given alkyl halide will be 4-(bromomethyl)heptane. 

Rules for Naming Alkyl Halides

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Alkyl halides are organic compounds with halogen group bonded on parent chain of carbon atoms of organic compound. Alkyl halides may have one or more halogen groups bonded at different locations of carbon chain. They can be considered as derivatives of alkanes with one of hydrogen replaced by halogen group.

 Halo Alkane

On the basis of presence of halogen group on parent chain, alkyl halides can be classified as primary, secondary and tertiary alkyl halides. Primary alkyl halide carries the halogen atom on a primary carbon atom which is bonded to one other alkyl group. For example ethyl bromide $(CH_{3}-CH_{2}-Br)$ and propyl bromide $(CH_{3}-CH_{2}-CH_{2}-Br)$ are primary alkyl halides. In a secondary alkyl halide the carbon with the halogen attached is bonded with two other same or different alkyl groups. 

Secondary Alkane

Unlike secondary, tertiary alkyl halides have halogen bonded on a tertiary carbon atom of parent chain. 
Tertiary Alkane

We know that group-17 elements are commonly called as halogens so alkyl halides can have any halogen bonded on carbon atom and prefix will come accordingly.

Like naming of alkanes, in the nomenclature of alkyl halides also, the longest chain will determine the root word with suffix –ane (If all carbon atoms are bonded with single bonds). The name of halogen defines the halide prefix with lowest possible location.
Here the longest carbon chain consists of 4 carbon atoms which are bonded with single covalent bonds. Here –Cl group is bonded on one of the terminal of carbon chain so we have to give numbering from that side only.

 Chloro Alkane

Hence the name of compound must be 1-chlorobutane or alkyl bromide. The general formula of alkyl halides is R-X; here R represents alkyl group and X stands for halogen. So in above compound, butyl is R group and X is Cl group.

Naming Alkyl Halides Examples

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IUPAC provides a systematic naming system for all organic compounds. There are some sets of rules which provide a general pattern of naming and help us to predict the name of any unknown chemical compound with the help of its formula. For example alkyl halides are organic compounds with halogen bonded on carbon chain. The systematic name of alkyl halide is haloalkanes here halo represents the halogen group and alkane indicates the carbon chain. 
According to IUPAC rules we have to approach the longest chain with functional group at least position. The longest chain consists of 6 C atoms so root word will be hex- with –ane suffix. The methyl group is positioned at 4 and I- at 3 positions. 

Hence the systematic name of molecule will be 3-iodo-4-methylhexane or 1-ethyl-2-methylbutyl iodide. 

Tertiary Alkane

Here the alkyl group of R-X (alkyl halide) is a tert-butyl group with bromine as halogen so the name must be tert-butyl bromide. The longest chain contains 3 C atoms so prop- must be root word with –ane as suffix (as all carbon atoms are bonded with single covalent bond). The bromine group will come as prefix bromo at C-2 and one side chain of methyl group is also at same carbon atom. Hence systematic name of given compound would be 2-bromo-2-methylpropane. 

Naming Alkyl Halides With Double Bond

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If any other functional group is present in alkyl halide, that functional group will get priority and gets lowest number compare to halo group. The functional group will indicate the suffix and halo group will be part of prefix. For example: 

$CH_{2}$ = $CHCH_{2}CH_{2}Br$

Here the longest chain consists of 4 carbon atoms so But- must be root word for this. The presence of one double bond will add suffix –ene. Here bromo group is substituent so it will be part of prefix as bromo-. For numbering the longest chain we should give priority to main functional group that is double bond in this molecule so it requires numbering from the left to make the alkene group at 1- position and bromine at 4-position. Hence the systematic name of given compound would be 4-bromobut-1-ene. 
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