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Soumen Goswami

Alkenes and Alkynes

Alkenes and alkynes are hydrocarbons which respectively have carbon-carbon double bond and carbon-carbon triple bond functional groups. The molecular formulas of these unsaturated hydrocarbons reflect the multiple bonding of the functional groups:

Alkane	R?CH₂?CH₂?R	C_nH_2n+2	This is the maximum H/C ratio for a given number of carbon atoms.
Alkene	R?CH=CH?R	C_nH_2n	Each double bond reduces the number of hydrogen atoms by 2.
Alkyne	R?C?C?R	C_nH_2n-2	Each triple bond reduces the number of hydrogen atoms by 4.

For example, consider compounds having the formula C₅H₈. The formula of the five-carbon alkane pentane is C₅H₁₂ so the difference in hydrogen content is 4. This difference suggests such compounds may have a triple bond, two double bonds, a ring plus a double bond, or two rings. Some examples are shown here, and there are at least fourteen others!

IUPAC Rules for Alkene and Cycloalkene Nomenclature

1. The ene suffix (ending) indicates an alkene or cycloalkene.
2. The longest chain chosen for the root name must include both carbon atoms of the double bond.
3. The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
4. The smaller of the two numbers designating the carbon atoms of the double bond is used as the double bond locator. If more than one double bond is present the compound is named as a diene, triene or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.
5. In cycloalkenes the double bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.
6. Substituent groups containing double bonds are:
H₂C=CH? Vinyl group
H₂C=CH?CH₂? Allyl group

IUPAC Rules for Alkyne Nomenclature

1. The yne suffix (ending) indicates an alkyne or cycloalkyne.
2. The longest chain chosen for the root name must include both carbon atoms of the triple bond.
3. The root chain must be numbered from the end nearest a triple bond carbon atom. If the triple bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
4. The smaller of the two numbers designating the carbon atoms of the triple bond is used as the triple bond locator.
5. If several multiple bonds are present, each must be assigned a locator number. Double bonds precede triple bonds in the IUPAC name, but the chain is numbered from the end nearest a multiple bond, regardless of its nature.
6. Because the triple bond is linear, it can only be accomodated in rings larger than ten carbons. In simple cycloalkynes the triple bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.
7. Substituent groups containing triple bonds are:
HC?C? Ethynyl group
HC?CH?CH₂? Propargyl group

Benzene Derivatives

The nomenclature of substituted benzene ring compounds is less systematic than that of the alkanes, alkenes and alkynes. A few mono-substituted compounds are named by using a group name as a prefix to "benzene", as shown by the combined names listed below. A majority of these compounds, however, are referred to by singular names that are unique. There is no simple alternative to memorization in mastering these names.

Two commonly encountered substituent groups that incorporate a benzene ring are phenyl, abbreviated Ph-, and benzyl, abbreviated Bn-. These are shown here with examples of their use. Be careful not to confuse a phenyl (pronounced fenyl) group with the compound phenol (pronounced feenol). A general and useful generic notation that complements the use of R- for an alkyl group is Ar- for an aryl group (any aromatic ring).

When more than one substituent is present on a benzene ring, the relative locations of the substituents must be designated by numbering the ring carbons or by some other notation. In the case of disubstituted benzenes, the prefixes ortho, meta & para are commonly used to indicate a 1,2- or 1,3- or 1,4- relationship respectively. In the following examples, the first row of compounds show this usage in red. Some disubstituted toluenes have singular names (e.g. xylene, cresol & toluidine) and their isomers are normally designated by the ortho, meta or para prefix. A few disubstituted benzenes have singular names given to specific isomers (e.g. salicylic acid & resorcinol). Finally, if there are three or more substituent groups, the ring is numbered in such a way as to assign the substituents the lowest possible numbers, as illustrated by the last row of examples. The substituents are listed alphabetically in the final name. If the substitution is symmetrical (third example from the left) the numbering corresponds to the alphabetical order.

Here are a few sites where you can learn nomenclature

http://www.chem.ucalgary.ca/courses/351/orgnom/index.html

http://www.acdlabs.com/iupac/nomenclature/

http://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry

http://www.chem.qmul.ac.uk/iupac/

Alkenes and Alkynes

IUPAC Rules for Alkene and Cycloalkene Nomenclature

IUPAC Rules for Alkyne Nomenclature

Benzene Derivatives

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