1. Alkanes: Nomenclature
Although many different types of nomenclature, or naming systems, were employed
in the past, today only the International Union of Pure and Applied Chemistry
(IUPAC) nomenclature is acceptable for all scientific publications. In this system, a
series of rules has been created that is adaptable to all classes of organic compounds.
For alkanes, the following rules apply.
1. Identify the longest continuous chain of carbon atoms. The parent name of the
alkane is the IUPAC‐assigned name for the alkane of this number of carbon atoms (see
Table ). Thus, if the longest chain of carbon atoms has six carbon atoms in it, the
parent name for the compound is hexane.
2. Identify the substituent groups attached to the parent chain. A substituent group is
any atom or group that has replaced a hydrogen atom on the parent chain.
3. Number the continuous chain in the direction that places the substituents on the
lowest‐numbered carbon atoms.
4. Write the name of the compound. The parent name is the last part of the name. The
name(s) of the substituent group(s) and the location number(s) precede the
parent name. A hyphen separates the number associated with the substituent from
its name. If two substituents are on the same carbon of the parent chain, the
number of the carbon they are attached to is written before eachsubstituent name.
If the two substituents are identical, the numbers are both written before the
substituent name, and the prefix “di” is added to the name. Substituent group names
are placed in alphabetical order.
Alkenes are normally named using the IUPAC system. The rules for alkenes are
similar to those used for alkanes. The following rules summarize alkene
nomenclature.
2. 1. Identify the longest continuous chain of carbon atoms that contains the carbon‐
carbon double bond. The parent name of the alkene comes from the IUPAC name for
the alkane with the same number of carbon atoms, except the ‐ane ending is changed
to ‐ene to signify the presence of a double bond. For example, if the longest continuous
chain of carbon atoms containing a double bond has five carbon atoms, the
compound is a pentene.
2. Number the carbon atoms of the longest continuous chain, starting at the end closest to the
double bond. Thus, is numbered from right to left, placing the double bond between the second
and third carbon atoms of the chain. (Numbering the chain from left to right incorrectly
places the double bond between the third and fourth carbons of the chain.)
3. The position of the double bond is indicated by placing the lower of the pair of numbers
assigned to the double‐bonded carbon atoms in front of the name of the alkene. Thus, the
compound shown in rule 2 is 2‐pentene.
4. The location and name of any substituent molecule or group is indicated. For example, is 5‐
chloro‐2‐` hexene.
5. Finally, if the correct three‐dimensional relationship is known about the groups attached to
the double‐ bonded carbons, the cis or trans conformation label may be assigned. Thus, the
complete name of the compound in rule 4 (shown differently here) is cis‐5‐chloro‐2‐hexene.
3.
4. Applying the four nomenclature rules to the following compound
results in the name 2‐chloro‐3‐methylpentane. Notice that the parent name comes from
the longest continuous carbon chain, which begins with the carbon of the CH 3 group at
the bottom of the structural formula (a) and goes to the carbon of the CH 3 group on
the top right side of the formula (b). This chain contains five carbon atoms, while the
straight chain of carbons from left to right contains only four carbons. Starting the
numbering from the top right carbon of the CH 3 group ( b) leads to 2,3 substitution,
while numbering from the bottom right side CH 3 carbon ( a) leads to 3,4 substitution
(which is incorrect). This alkane is referred to as a branched‐chain alkane because it
contains an alkyl group off of the main chain.
Applying the IUPAC nomenclature rules to a more complex alkane molecule
results in the name 5‐chloro‐2‐hydroxy‐4‐propylheptane. Notice that the names of the
substituent groups are in alphabetical order.
Finally, here is an example of a compound with a repeating substituent group.
5. Isomerism in Branched Alkanes
In n-alkanes, no carbon is bonded to more than two other carbons, giving rise to a linear chain.
When a carbon is bonded to more than two other carbons, a branch is formed. The smallest
branched alkane is isobutane. Notice that isobutane has the same molecular formula, C 4 H 10 , as n-
butane but has a different structural formula. Two different molecules which have the same
molecular formula are isomers. Isomers which differ in the connectivity of bonds are constitutional
isomers, or structural isomers. Isobutane is a constitutional isomer of n-butane. The prefix"iso"
indicates that branches off of the central carbon are equivalent.
Figure %: The constitutional isomers butane and isobutane.
n-butane and isobutane are the only constitutional isomers of C4 H 10 . Pentane, C 5 H 12 , has three
while hexane, C 6 H 14 , has five.
Figure %: Constitutional isomers of pentane and hexane.
Nomenclature of Alkanes
6. Isobutane, neopentane, etc. are trivial names that arise from common usage. As you can see, the
number of isomers increases rapidly for larger alkanes. It would be impractical to give trivial names
to every isomer. What is needed is a systematic, easy-to-use method of naming that works for even
the most complex of molecules. Such a name should unambiguously identify the structural formula
of the named molecule. This system is IUPAC nomenclature, devised by the International Union of
Pure and Applied Chemists.
The IUPAC system considers molecules in terms of a parent hydrocarbon chain with substituents
attached to it. The parent is the longest continuous carbon chain in the compound, and the base
name of the compound is the alkane that corresponds to the parent chain. Then, consecutively
number the carbons of the parent chain in such a way that the substituents are attached to carbons
with lower numbers. The name of the compound is the parent alkane prefixed by its substituents and
their position numberings.
Figure %: Correct and incorrect IUPAC names for isopentane.
The -CH3 group is called a methyl group. In general, alkyl substituents are derived from the
corresponding alkanes by replacing the -ane suffix with -yl. These substituents are used so
frequently that they are given abbreviated names. For instance, methyl groups can be abbreviated
as -Me. In some instances, the exact nature of the substituent is unimportant. In such cases the
notation -R can be used to denote a radical group, a general substituent that can be any organic
component.
7. Figure %: Common alkyl substituents and their abbreviated names.
Sometimes there is more than one possible choice of parent chains. In such cases,choose the
parent chain whose substituents are least substituted.
Figure %: How to break ties when choosing the parent chain.
Classification of Carbon Substitution
A particular carbon atom is often described in terms of its degree of branching. When a carbon is
attached to only one other carbon atom, it is said to be primary( 1o
). Similarly carbons attached to
two, three, and four other carbon atoms are secondary( 2o
), tertiary( 3o
), and quaternary( 4o
),
respectively. Methane is not attached to any other carbons, so it forms its own category in this
classification system.
8. Figure %: Carbon substitution.
Nomenclature of Alkenes and Alkynes
Alkenes and alkynes are named with the same prefixes as their alkane counterparts but their
suffixes are changed to -ene and - yne, respectively. The position of the double or triple bond within
the carbon chain is denoted by the position of the carbon within the bonded pair that has the lower
numbering. The numbering of the parent chain should also be oriented in such a way that the double
bond receives the lowest numbering possible: A hexene with its double bond at the end should be 1-
hexene, not 2-, 5-, or 6-hexene.
Figure %: Naming alkenes and alkynes.
Alkenes have a general molecular formula C n H 2n and alkynes have a genera...molecular formula
of C n H (2n - 2) . This trend makes sense because the presence of each pi ( Π ) bond removes
two σ bonds available for bonding to hydrogens. We will see that there are chemical reactions that
add hydrogens to C-CΠ bonds and turn alkenes and alkynes into alkanes, and that there are
reactions to reverse the transformation and produce alkenes and alkynes from alkanes. An alkane is
said to be a saturated hydrocarbon because no more hydrogens can be added to the molecule.
9. Conversely, alkenes and alkynes are unsaturated hydrocarbons.The number of pairs of hydrogens
that a hydrocarbon is missing from (2n + 2) is its unsaturation number. A molecule's unsaturation
number can be calculated from its molecular formula C n H m :
UnsaturationNumber = ((2n + 2)–m)
Even though it's impossible to add hydrogens to cyclic alkanes, cyclic alkanes
are still considered "unsaturated" in this sense because they have molecular
formulas C n H 2n . In general, a molecule's unsaturation number is equal to the
sum of its number of Π bonds and rings.
Cis-trans Isomerism in Alkenes
Other types of isomerism exist besides constitutional isomerism. Two molecules can have the same
atomic connectivities and yet have different spatial arrangements of atoms. Such isomers are
stereoisomers. Stereoisomerism takes many forms and will be discussed in great detail in the next
chapter.
Alkenes exhibit one form of stereoisomerism. To understand howalkenes can form stereoisomers,
recall that the C=C double bond consists of a σ bond between the atoms and a Π bond that lies
above and below the plane of the molecule. The strength of the Π bond depends directly on the
degree of physical overlap between adjacent p-orbitals. This implies that it is impossible to rotate
about the double bond without breaking the Π bond completely. This requires a great deal of energy
and does not occur to any appreciable extent at room temperature.
10. Figure %: Illustrating the resistance of double bonds to rotation.
This lack of rotational freedom explains why the following two molecules cannot readily interconvert.
These two molecules are stereoisomers because they have the same atomic connectivity and yet
are different. The isomer in which both methyl substituents are on the same side of the double bond
is called cis, meaning "same". The other isomer with substituents on opposite sides of the double
bond is calledtrans, which means "across". This particular type of stereoisomerism is called cis-
trans isomerism, or geometrical isomerism. As we'll see, cyclic alkanes can also exhibit cis-
trans isomerism.
11. ALCOHOLS, PHENOL and ETHERS
Reference: McMurry Ch 8 George et al Ch 2.2 � 2.4
Nomenclature
Alcohols: Rules for naming alcohols follow the guidelines already given for alkanes,
in summary
The number of carbon atoms in the longest carbon chain containing the OH
group gives the stem
Use a prefix to identify the position of the carbon carrying the OH and a suffix
of -ol. Number from the end of the chain closest to the alcohol group
Use numbers and di-, tri- etc as appropriate
If a molecule contains a multiple bond as well as an alcohol group, give the
carbon with the OH group attached the lowest possible number
Alcohols may be classified as primary (1 ), secondary (2 ), tertiary (3 )
depending on whether the carbon atom carrying the OH is attached to 1 other
carbon group, 2 other carbon groups or 3 other carbon groups respectively.
12. Examples:
Ethers: Two ways of naming ethers
The alkyl (or aryl) groups attached to the -O- are named in alphabetical order as
two separate words and the word ether added.
If both of the groups attached to the ether oxygen are the same, the ether name
is simplified by using the prefix "di-" with the name of the
group e.g. CH3OCH3 is called dimethyl ether.
Alternatively, ethers may be named as alkoxy derivatives of alkanes. In this
method of naming, the longest continuous alkyl chain forms the stem of the
ether name and the alkoxy group is named as a substituent on the alkane
backbone.
Examples:
Structure and properties
13. The oxygen atom of an alcohol is sp3
hybridised and has two non bonding pairs
of electrons
The O-H bond of alcohols is strongly polarised and hydrogen bonding occurs in
much the same way as in water molecules
As a consequence, alcohols have relatively high boiling points compared to
other organic compounds of a similar molecular weight and alcohols
(particularly the lower members of the series) are significantly more water
soluble than other classes of organic compounds which are not capable of
hydrogen bonding
Phenol also shows hydrogen bonding and is partially soluble in water. However
ethers are not hydrogen bond donors and so are not soluble in water
Name Structure Molecular Mass bp ( C) Water
Solubility
Ethanol CH3CH2OH 46 78 3
Dimethyl ether CH3OCH3 46 -24 7
Propane CH3CH2CH3 44 -42 7
1-butanol CH3CH2CH2CH2OH 74 117 (3 )
Diethyl ether CH3CH2OCH2CH3 74 35 7
Phenol C6H5OH 94 182 (3 )
14. Alcohols absorb radiation strongly ~ 3500 cm-1 in the infrared region
Reactions of Alcohols, Phenol and Ethers
1. Acid-base reaction of alcohols and phenol
Alcohols are very weak acids (somewhat weaker than water) but may loose
H+ from the OH group if sodium or a sufficiently strong base is present
Phenol is more acidic than alcohols and H+ may be removed with sodium
hydroxide solution. It is less acidic than carboxylic acids.
Example:
Relative acidities
pKa React with
Na
React with
OH-
React with
HCO3
-
Ethanol CH3CH2OH 16.0 X X
Phenol C6H5OH 9.9 X
Acetic Acid CH3COOH 4.8
15. 2. Oxidation of alcohols
Alcohols are oxidised by a variety of oxidising agents, e.g. potassium
permanganate in either acidic or basic solution (KMnO4/H+
or KMnO4/OH�) or
potassium dichromate in acidic solution (K2Cr2O7/H+
).
The product of alcohol oxidation depends on whether the starting alcohol is a
primary, secondary or tertiary alcohol.
Oxidation of methanol is unique amongst alcohols as the eventual products of
methanol oxidation are water and carbon dioxide.
16. 3. Nucleophilic substitution of alcohols
Concentrated HX acids (X = Cl, Br, or I) directly convert alcohols to alkyl
halides
The reaction takes place in two steps: protonation followed by substitution
Protonation converts the R-OH group to R-OH2
+ which can then loose H2O
Substitution of the halide ion for the protonated -OH group affords an alkyl
halide
Example
17. 4. Elimination of water from alcohols
Alcohols can also undergo an elimination reaction to form an alkene. H2O is
eliminated from the alcohol so the reaction is also called a dehydration reaction
This requires a dehydrating reagent, typically concentrated H2SO4
The OH is removed and a hydrogen from the adjacent carbon atom
Where there is a choice of hydrogens that can be eliminated, the one that results
in the most substituted alkene is removed
Example:
5 ETHERS
Ethers tend to be unreactive and consequently make good solvents.
Some biologically active compounds containing the alcohol
18.
19.
20. Principles of Organic Nomenclature
The additive operation involves the formal assembly of a structure from its component parts
without loss of any atoms or groups. This can be expressed in various ways as follows:
R-1.2.3.1 By use of an additive prefix
Examples to R-1.2.3.1
Naphthalene 1,2,3,4-Tetrahydronaphthalene
(hydro = addition of one H atom)
(homo = addition of a CH2 (methylene) group, in this case to
expand a ring)
Note: Coordination nomenclature, used extensively in nomenclature for inorganic compounds, is
an additive operation.
Examples to R-1.2.3.1
Dichlorobis(triethylphosphine)platinum
(chloro = addition of one Cl atom;
triethylphosphine = addition of one (C2H5)3P ligand group)
R-1.2.3.2 By use of an additive suffix
Examples to R-1.2.3.2
Pyridine
21. Pyridinium
(-ium = addition of one H+)
R-1.2.3.3.1 With the name of a neutral parent structure
Examples to R-1.2.3.3.1
Trimethylarsane Trimethylarsane sulfide
Styrene
Styrene oxide
R-1.2.3.3.2 With one or more substituent prefix ("radical") name(s) (formerly called
radicofunctional nomenclature) . Here the separate word is a class or subclass name
representing the characteristic group or the kind of characteristic group to which the substituents
("radicals") are linked.
Examples to R-1.2.3.3.2
R-1.2.3.4 By connecting the names of the components of an addition compound with a dash
(long hyphen)
Example to R-1.2.3.4
