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==Overview==<br />
[[image:cyclobutane.png|frame|right|cyclobutane]]<br />
'''Cycloalkanes''' (also called naphthenes) are chemicals with one or more [[hydrogen]] rings to which [[carbon]] atoms are attached according to the formula C<sub>n</sub>H<sub>2n</sub>. Cycloalkanes with a single ring are named analogously to their normal [[alkane]] counterpart of the same carbon count: [[cyclopropane]], [[cyclobutane]], [[cyclopentane]], [[cyclohexane]], etc. The larger cycloalkanes, with greater than 20 <!--this could be less, now that I think about it the information in the [[paraffin]] article could be wrong, or not apply to the ring varieties --> carbon atoms are typically called cycloparaffins.<br />
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Cycloalkanes are classified into small, normal and bigger cycloalkanes, where cyclopropane and cyclobutane are the small ones, cyclopentane, cyclohexane, cycloheptane are the normal ones, and the rest are the bigger ones.<br />
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== Nomenclature ==<br />
:''See also: [[IUPAC nomenclature]]''<br />
[[Image:Bicyclo2-1-1heptane.gif|thumb|right|bicyclo[2,2,1]heptane]]<br />
The naming of polycyclic alkanes such as [[bicyclic]] alkanes and [[spiro compound|spiro]] alkanes is more complex, with the base name indicating the number of carbons in the ring system, a prefix indicating the number of rings (eg, "bicyclo"), and a numeric prefix before that indicating the number of carbons in each part of each ring, exclusive of vertices. For instance, a bicyclooctane which consists of a six-member ring and a four member ring, which share two adjacent carbon atoms which form a shared edge, is [4.2.0]-bicyclooctane. That part of the six-member ring, exclusive of the shared edge has 4 carbons. That part of the four-member ring, exclusive of the shared edge, has 2 carbons. The edge itself, exclusive of the two vertices that define it, has 0 carbons.<br />
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The group of cycloalkanes are also known as '''naphthenes''', as they are compounds of [[petroleum]] or [[naphtha]].<br />
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== Properties ==<br />
Cycloalkanes are similar to alkanes in their general physical properties, but they have higher [[boiling point|boiling points]], [[melting point|melting points]], and [[density|densities]] than alkanes. This is due to stronger [[London forces]] because the ring shape allows for a larger area of contact. Cycloalkanes exhibit almost the same degree of unreactivity as alkanes, due to only containing unreactive C-C and C-H bonds; however, the ring strain (see below) can cause cycloalkanes to be more reactive.<br />
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== Ring strain ==<br />
The carbon atoms in cycloalkanes are ''sp''<sup>3</sup> [[orbital hybridization|hybridized]] and therefore a deviation from the ideal tetrahedral bond angles of 109.47 degrees causes an increase in potential energy and an overall destabilizing effect. Eclipsing of hydrogen atoms is an important destabilizing effect as well. The '''strain energy''' of a cycloalkane is the theoretical increase in energy caused by the compound's geometry, and is calculated by comparing the experimental [[standard enthalpy change of combustion]] of the cycloalkane with the value calculated using [[average bond energies]].<br />
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Ring strain is highest for [[cyclopropane]], in which the carbon atoms form a triangle and therefore have 60 degree C-C-C bond angles. There are also three pairs of eclipsed hydrogens. The ring strain is calculated to be around 120 kJ/mol. [[Cyclobutane]] has the carbon atoms in a puckered square with approximately 90 degree bond angles; by "puckering" it reduces the eclipsing interactions between hydrogen atoms. Its ring strain is slightly less, at around 110 kJ/mol. For a theoretical planar [[cyclopentane]] the C-C-C bond angles would be 108 degrees, very close to the tetrahedral angle. Actual cyclopentane molecules are puckered but this only changes the bond angles slightly so that angle strain is relatively small. The eclipsing interactions are also reduced, leaving a ring strain of about 25 kJ/mol.<br />
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In [[cyclohexane]] the ring strain and eclipsing interactions are negligible because the puckering of the ring allows ideal tetrahedral bond angles to be achieved. As well, in the most stable ''chair form'' of cyclohexane, axial hydrogens on adjacent carbon atoms are pointed in opposite directions, virtually eliminating eclipsing strain.<br />
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After cyclohexane, the molecules are unable to take a structure with no ring strain, resulting in an increase in strain energy, which peaks at 9 carbons (around 50 kJ/mol). After that, strain energy slowly decreases until 12 carbon atoms, where it drops significantly; at 14 another significant drop occurs and the strain is on a level comparable with 10 kJ/mol. After 14 carbon atoms, sources disagree on what happens to ring strain, some indicating that it increases steadily, others saying that it disappears entirely.<br />
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== Reactions ==<br />
The simple and the bigger cycloalkanes are very stable, like [[alkanes]], and their reactions for example [[radical substitution|radical chain reactions]], are like alkanes.<br />
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The small cycloalkanes - particularly cyclopropane - have a lower stability due to [[Baeyer strain theory|Baeyer strain]] and [[ring strain]]. They react similarly to [[alkenes]], though they don't react in [[electrophilic addition]] but in [[nucleophilic aliphatic substitution]]. These reactions are ring opening reactions or ring cleavage reactions of [[alkyl cycloalkane]]s. Cycloalkanes can be formed in a [[Diels-Alder reaction]] followed by a [[catalytic hydrogenation]].<br />
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==References==<br />
{{GoldBookRef| title=Cycloalkanes | file=C01497 | year=1995}}<br />
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==External Links==<br />
*[http://www.britannica.com/eb/article-79615/chemical-compound#615967.hook "Cycloalkanes"] at the online [[Encyclopædia Britannica]]<br />
*[http://www.westminster.edu/staff/Kailbw/CH2b.pdf "Cycloalkanes"] by the Chemistry staff at [[Westminster College, Pennsylvania|Westminster College]]<br />
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