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Chapter 2: Alkanes, Alkenes, AlkynesWe previously described that, carbon forms strong covalent bonds to other carbons and to other elements such as hydrogen, oxygen, nitrogen and sulfur. This accounts for the vast variety of organic compounds possible. This also accounts for the physical and chemical properties of these compounds (melting point, solubility, polarity) Organic compounds are grouped into functional group families. A functional group is a specific grouping of atoms which exhibit a characteristic reactivity when treated with certain reagents. We can discuss the chemical reactivity according to classes of function groups not to individual compounds. An instrumental technique called infrared (IR) spectroscopy is used to determine the presence of specific functional groups (simulation lab). Classes of hydrocarbons Hydrocarbons are compounds that contain only carbon and hydrogen and are divided into two main classes: aliphatic hydrocarbons and aromatic hydrocarbons (natural source origin). Hydrocarbons: Representative Alkanes, Alkenes Alkynes, and Aromatic Compounds Aliphatic hydrocarbons could be devided into subgroups Alkanes contain only carbon-carbon single bonds Alkenes contain one or more carbon-carbon double bonds Alkynes contain one or more carbon-carbon triple bonds Aromatic hydrocarbons contain benzene-like stable structures (discussed later).
Hydrocarbons could also devided into Saturated hydrocarbons: contain only carbon-carbon single bonds e.g. alkanes
Unsaturated hydrocarbons: contain double or triple carbonCapable of reacting with H2 to become saturated
carbon bonds e.g. alkene, alkynes, aromatics A functional group is the structural unit responsible for a given molecules reactivity under a particular set of conditions. One of the hydrogen atoms of ethane is replaced by chlorine. This replacement of hydrogen by chlorine is a characteristic reaction of all alkanes.
In the general equation the functional group (-H) is shown explicitly while the remainder of the alkane molecule is abbreviated as R.
A hydrogen atom in one alkane is very much like the hydrogen of any other alkane in its reactivity toward chlorine. Our ability to write general equations such as the one shown illustrates why the functional group approach is so useful in organic chemistry. A hydrogen atom is a functional group in arenes, and we represent arenes as ArH to reflect this. In case of alkene and alkynes as hydrocarbons the function group is mainly the double and triple bonds, respectively. Function groups in Organic Chemistry The most important function groups in Organic Chemistry are listed in the following tables
Classes of carbonyl group function groups Methane is A component of the atmosphere of many planets Major component of natural gas Produced by primitive organisms called methanogens Represent 98% of natural gas in Egypt. Ethane (C2H6) and propane (CH3CH2CH3) are second In all alkanes: as the number of carbon
found in mud, sewage and cows stomachs
(10 %) and third (5 %), most abundant component of natural gas. atoms increases, so does the boiling point.
Alkane isomers Beginning with C4H10, however, constitutional isomers are possible; two alkanes have this particular molecular formula (n-butane and isobutene).
Three isomeric alkanes have the molecular formula C5H12 (n-pentane, isopentane and neopentane).
The following table shows the number of constitutional isomers of alkanes
The challenge now is 1. 2. How can we tell when we have written all the possible isomers How can we name alkanes so that each one has a unique name? corresponding to a particular molecular formula?
Write all possible constitutional isomers of C6H14
Iupac Nomenclature of alkanes The IUPAC rules name branched alkanes as substituted derivatives of the unbranched alkanes listed in Table.
Step 1: Pick out the longest continuous carbon chain, and nd the IUPAC name in Table that corresponds to the unbranched alkane having that number of carbons. This is the parent alkane from which the IUPAC name is to be derived. Step 2: Identify the substituent groups attached to the parent chain. The parent pentane chain bears a methyl (CH3) group as a substituent. Alkyl groups Substituent alkyl goup lacks one of the hydrogen substituents of an alkane. A methyl group (CH3-) is an alkyl group derived from methane (CH4). Unbranched alkyl groups in which the point of attachment is at the end of the chain are named in IUPAC nomenclature by replacing the -ane endings of Table by -yl.
Step 3 Number the longest continuous chain in the direction that gives the lowest number to the substituent group at the rst point of branching. The numbering scheme
Step 4 Write the name of the compound. The parent alkane is the last part of the name and is preceded by the names of the substituent groups and their numerical locations (locants).
IUPAC names of highly branched alkanes By combining the basic principles of IUPAC notation with the names of the various alkyl groups, we can develop systematic names for highly branched alkanes. Well start with the following alkane, name it, then increase its complexity by successively adding methyl groups at various positions.
4-ethyloctane Name the substituent alkyl groups in alphabetical order. Each substituent with an individual locant number
4-ethyl-3-methyloctane Use the prefixes di, tri and tetra for multiple substituent same alkyl group, each substituent with an individual locant number separated by a comma. Note the alphabetical order doesn't include the prefixes.
Note1 In this case, the longest continuous chain has ve carbon atoms; the compound is named as a derivative of pentane. The key word here is continuous. It does not matter whether the carbon skeleton is drawn in an extended straight-chain form or in one with many bends and turns.
Longest continuous C's is 10 (decane ) An additional feature of IUPAC nomenclature that concerns the direction of numbering is called the first point of difference rule. Consider the two directions in which the following alkane may be numbered:
When two chains of equal length compete to be parent, choose the
chain with the greatest number of substituents
parent alkane 9C's
parent alkane 9C's 5 substituents
When two different numbering schemes give equivalent sets of locants,
choose the direction that gives the lower locant to the group in alphabetical
3-ethyl-4-methyl hexane Systematic names of branched alkyl groups Branched alkyl groups are according to the following rules 1. 2. 3. 4. Number the carbon atoms beginning at the point of attachment,Assign a basis name according to the number of carbons in the corresponding
proceeding in the direction that follows the longest continuous chain.unbranched alkane. Drop the ending -ane and replace it by -yl.
List the substituents on the basis group in alphabetical order using Locate the substituents according to the numbering of the main chain
replicating prefixes when necessary. described in step 1.
naming branched alkyl substituent
parent alkane 9C's
Cycloalkanes cycloalkanes are named, under the IUPAC system, by adding the prefix cyclo- to the name of the unbranched alkane with the same number of carbons as the ring. Substituent groups are identified in the usual way. Their positions are specified by numbering the carbon atoms of the ring in the direction that gives the lowest number to the substituents at the first point of difference.
When the ring contains fewer carbon atoms than an alkyl group attached to it, the compound is named as an alkane, and the ring is treated as a cycloalkyl substituent:
Sources of alkanes and cycloalkanes natural gas is especially rich in methane and also contains Petroleum is a liquid mixture containing hundreds of ethane and propane substances, including approximately 150 hydrocarbons, roughly half of which are alkanes or cycloalkanes Distillation of crude oil gives a number of fractions. Modern petroleum refining involves more than distillation, Cracking. It is a process that involves the cleavage of carbon
however, and includes two major additional operations: carbon bonds of high molecular weight hydrocarbons induced by heat (thermal cracking) or with the aid of certain catalysts
(catalytic cracking) to give more volatile, lower-molecular-weight hydrocarbons useful as automotive fuels and as a source of petrochemicals. Reforming. It is a process that involves conversion of unbranched hydrocarbons in petroleum to aromatic hydrocarbons and highly branched alkanes, both of which show less tendency for knocking (preignition).
Physical properties of alkanes and cycloalanes The boiling points of unbranched alkanes increase with the number of carbon atoms. Figure also shows that the boiling points for 2-methyl-branched alkanes are lower than those of the unbranched isomer.
A substance exists as a liquid rather than a gas because attractive forces between molecules (intermolecular attractive forces) are greater in the liquid than in the gas
phase. Attractive forces between neutral species (atoms or molecules, but not ions) are referred to as van der Waals forces and may be of three types: 1. 2. 3. dipole-dipole dipole/induced-dipole induced-dipole/induced-dipole
in order to vaporize a substance, enough energy must be added to overcome them. Most alkanes have no measurable dipole moment, and therefore the only van der Waals force to be considered is the induced- dipol