physical properties of alkanes-concept of homology




The series of continuous-chain alkanes, CH,(CH,),-,CH, , shows a remarkably smooth gradation of physical properties (see Table 3.3 and Figure 3-2). As you go up the series, each additional CH, group contributes a fairly constant increment to the boiling point and density and, to a lesser extent, to the melting point. This makes it possible to estimate the properties of an unknown member of the series from those of its neighbors. For example, the boiling points of hexane and heptane are 69" and 98", respectively; a difference in structure of one CH, group therefore makes a difference in boiling point of 29". This places the boiling point of the next higher member, octane, at 98" + 29", or 127", which is close to the actual boiling point of 126".

Members of a group of compounds with similar chemical structures and graded physical properties and which differ from one another by the number of atoms in the structural backbone, such as the n-alkanes, are said to constitute a homologous series. The concept of homology, when used to forecast the properties of unknown members of the series, works most satisfactorily for the higher-molecular-weight members. For these members, the introduction of additional CH, groups makes a smaller relative change in the overall composition of the molecule. This is better seen from Figure 3.2,

physical properties of alkanes-concept of homology

which shows how the boiling points and melting points of the homologous series of normal alkanes change with the number of carbons, a. See also Figure 3-3.

Branched-chain alkanes do not exhibit the same smooth gradation of physical properties as the n-alkanes. Usually, there is too great a variation in

physical properties of alkanes-concept of homology

physical properties of alkanes-concept of homology

molecular structure for regularities to be apparent. Nevertheless, in any one set of isomeric hydrocarbons, volatility increases with increased branching. This can be seen from the data in Table 3.4, in which are listed the physical

physical properties of alkanes-concept of homology

properties of the five hexane isomers; the most striking feature is the 19" difference between the boiling points of hexane and neohexane.



Frequently Asked Questions

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Ans: In the previous two chapters we have studied in some detail the properties of the two simplest saturated hydrocarbons, methane and ethane, and have shown how their simple derivatives are named using the rules of the International Union of Pure and Applied Chemistry (IUPAC rules). view more..
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Ans: The two simplest unsaturated compounds (those containing a multiple bond) are ethene (CH,=CH,) and ethyne (HCzCH). The generally lower stability of multiply bonded compounds arises from the restriction that only one electron pair can occupy a given orbital view more..
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Ans: In organic chemistry, the word structure has a specific meaning; It designates the order in which the atoms are joined to each other. A structure does not necessarily specify the exact shape of a molecule because rotation about single bonds could lead, even for a molecule as simple as ethane, to an infinite number of different arrangements of the atoms in space. view more..
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Ans: The series of continuous-chain alkanes, CH,(CH,),-,CH, , shows a remarkably smooth gradation of physical properties (see Table 3.3 and Figure 3-2). As you go up the series, each additional CH, group contributes a fairly constant increment to the boiling point and density and, to a lesser extent, to the melting point. view more..
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Ans: As a class, alkanes are singularly unreactive. The name saturated hydrocarbon (or " paraffin," which literally means " little affinity " [L. par(um), little, + afins, affinity1)arises because their chemical affinity for most common reagents may be regarded as saturated or satisfied. view more..
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Ans: An important and interesting group of hydrocarbons, known as cycloalkanes, contain rings of carbon atoms linked together by single bonds. The simple unsubstituted cycloalkanes of the formula (CH,), make up a particularly important homologous series in which the chemical properties change in a much more striking way than do the properties of the open-chain hydrocarbons, CH3(CH2),-,CH3. view more..
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Ans: In the early days of organic chemistry, when it was found that the alkenes, but not the alkanes, readily undergo addition reactions with substances such as halogens, hydrogen halides, sulfuric acid, and oxidizing agents, the chemical affinity of alkanes was said to be " saturated" while that of the alkenes was said to be " unsaturated." Now, even though we recognize that no chemical entity (even the noble gases such as helium and xenon) can surely be classified as saturated, the description of alkanes and alkenes as saturated and unsaturated is still commonly used. view more..
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Ans: In the homologous series of alkanes, isomerism first appears at the C, level, two compounds of formula C4H,, being known. These are structural isomers: view more..
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Ans: By convention, the configuration of complex alkenes is taken to correspond to the configuration of the longest continuous chain as it passes through the double bond. Thus the following compound is 4-ethyl-3-methyl-trans-3- heptene, despite the fact that two identical groups are cis with respect to each view more..
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Ans: We have previously examined briefly two addition reactions of ethene, the first member of the homologous series of alkenes. These were addition of hydrogen, catalyzed by surfaces of finely divided metals such as nickel, and the addition of bromine. view more..




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