· web view 5 ch3(ch2)3ch3 pentane c5h12 6 ch3(ch2)4ch3 hexane c6h14 7 ch3(ch2)5ch3 heptane...

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14
Alkanes
Alkanes-Introduction
· Recall that alkanes are aliphatic hydrocarbons having C—C and C— H bonds. They can be categorized as acyclic or cyclic.
· Acyclic alkanes have the molecular formula CnH2n+2 (where n = an integer) and contain only linear and branched chains of carbon atoms. They are also called saturated hydrocarbons because they have the maximum number of hydrogen atoms per carbon.
· Cycloalkanes contain carbons joined in one or more rings. Because their general formula is CnH2n, they have two fewer H atoms than an acyclic alkane with the same number of carbons.
Acyclic alkanes Having One to Five C Atoms
· All C atoms in an alkane are surrounded by four groups, making
them sp3 hybridized and tetrahedral, and all bond angles are 109.5°.
· The 3-D representations and ball-and-stick models for these alkanes indicate the tetrahedral geometry around each C atom. In contrast, the Lewis structures are not meant to imply any 3-D arrangement. Additionally, in propane and higher molecular weight alkanes, the carbon skeleton can be drawn in a variety of ways and still represent the same molecule.
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· Carbon atoms in alkanes and other organic compounds are classified by the number of other carbons directly bonded to them.
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· Hydrogen atoms are classified as primary (1°), secondary (2°), or tertiary (3°) depending on the type of carbon atom to which they are bonded.
· The maximum number of possible constitutional isomers increases dramatically as the number of carbon atoms in the alkane increases. For example, there are 75 possible isomers for an alkane having 10 carbon atoms, but 366,319 possible isomers for one having 20 carbons.
· The suffix “ane” identifies a molecule as an alkane.
Naming Alkanes
1 CH4 Methane CH4
2 CH3CH3 Ethane C2H6
3 CH3CH2CH3 Propane C3H8
4 CH3(CH2)2CH3 Butane C4H10
5 CH3(CH2)3CH3 Pentane C5H12
6 CH3(CH2)4CH3 Hexane C6H14
7 CH3(CH2)5CH3 Heptane C7H16
8 CH3(CH2)6CH3 Octane C8H18
9 CH3(CH2)7CH3 Nonane C9H20
10 CH3(CH2)8CH3 Decane C10H22
· By increasing the number of carbons in an alkane by a CH2 group, one obtains a “homologous series” of alkanes, as shown in Table
The CH2 group is called “methylene.”
Cycloalkanes
Cycloalkanes have molecular formula CnH2n and contain carbon atoms arranged in a ring. Simple cycloalkanes are named by adding the prefix cyclo- to the name of the acyclic alkane having the same number of carbons.
4.2 Cycloalkanes
The name of every organic molecule has 3 parts:
1. The parent name indicates the number of carbons in the longest continuous chain.
2. The suffix indicates what functional group is present.
3. The prefix tells us the identity, location, and number of substituents attached to the carbon chain.
Naming Substituents
· Carbon substituents bonded to a long carbon chain are called alkyl groups.
· An alkyl group is formed by removing one H atom from an alkane.
· To name an alkyl group, change the –ane ending of the parent alkane to –yl. Thus, methane (CH4) becomes methyl (CH3-) and ethane (CH3CH3) becomes ethyl (CH3CH2-).
Naming three- or four-carbon alkyl groups is more complicated because the parent hydrocarbons have more than one type of hydrogen atom. For example, propane has both 1° and 2° H atoms, and removal of each of these H atoms forms a different alkyl group with a different name, propyl or isopropyl.
Nomenclature of Alkanes
The systematic name of an alkane is obtained using the following rules:
IUPAC: International Union of Pure and Applied Chemistry
1. Determine the number of carbons in the longest continuous carbon chain. This chain is called the parent hydrocarbon. ( Find the parent hydrocarbon.the longest continuous chain of carbon atoms)
First, determine the number of carbons in the longest continuous chain.
2. The name of any alkyl substituent that hangs off the parent hydrocarbon is cited before the name of the parent hydrocarbon, together with a number to designate the carbon to which the alkyl substituent is attached.
Number the chain so that the substituent gets the lowest possible number.
3. If more than one substituent is attached to the parent hydrocarbon, the chain is numbered in the direction that will result in the lowest possible number in the name of the compound.
Substituents are listed in alphabetical order.
A number and a word are separated bya hyphen; numbers are separated by a comma.
If two or more substituents are the same, the prefixes “di,” “tri,” and “tetra”
are used to indicate how many identical substituents the compound has.
Substituents are listed in alphabetical order.
The prefixes di, tri, tetra, sec, and tert are ignored in alphabetizing substituent
groups, but the prefixes iso, neo, and cyclo are not ignored.
4. When both directions lead to the same lowest number for one of the substituents,
the direction is chosen that gives the lowest possible number to one of the remaining
substituents.
5. If the same substituent numbers are obtained in both directions, the first group cited receives the lower number.
6. If a compound has two or more chains of the same length, the parent hydrocarbon
is the chain with the greatest number of substituents.
In the case of two hydrocarbon chains with the same number of carbons,
choose the one with the most substituents.
7. Names such as “isopropyl,” “sec-butyl,” and “tert-butyl” are acceptable substituent
names in the IUPAC system of nomenclature, but systematic substituent
names are preferable. Systematic substituent names are obtained by numbering
the alkyl substituent starting at the carbon that is attached to the parent hydrocarbon.
The rules for naming cycloalkanes resemble the rules for naming acyclic alkanes:
1. In the case of a cycloalkane with an attached alkyl substituent, the ring is the parent
hydrocarbon unless the substituent has more carbon atoms than the ring. In
that case, the substituent is the parent hydrocarbon and the ring is named as a substituent.
There is no need to number the position of a single substituent on a ring.
If there is only one substituent on a ring, do not give that substituent a number
2. If the ring has two different substituents, they are cited in alphabetical order and
the number 1 position is given to the substituent cited first.
For rings with more than one substituent, begin numbering at one substituent and proceed around the ring to give the second substituent the lowest number.
With two different substituents, number the ring to assign the lower number to the substituents alphabetically.
3. If there are more than two substituents on the ring, they are cited in alphabetical
order. The substituent given the number 1 position is the one that results in a second substituent getting as low a number as possible. If two substituents have the same
low number, the ring is numbered—either clockwise or counterclockwise—in
the direction that gives the third substituent the lowest possible number. For
example, the correct name of the following compound is 4-ethyl-2-methyl-
1-propylcyclohexane, not 5-ethyl-1-methyl-2-propylcyclohexane:
Note the special case of an alkane composed of both a ring and a long chain. If the number of carbons in the ring is greater than or equal to the number of carbons in the longest chain, the compound is named as a cycloalkane.
Two contrasting examples—Naming compounds containing both a ring and a long chain of carbon atoms
Examples of cycloalkane nomenclature
Physical Properties of Alkanes

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