What is the difference between open chain alkanes and cycloalkanes?

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The open chain alkanes and cycloalkanes can be easily differentiate with the help of characteristic features.

Features of alkanes and cycloalkanes:

                 Open chain alkanes             Cycloalkanes
              
Open chain alkanes have an open chain of carbon atoms and have no cycle or ring in their structure.
Cycloalkanes have a closed chain of carbon atoms and have a cyclic structure.
 
Open chain alkanes have the general formula of CnH2n+2
Cycloalkanes have the general formula of CnH2n
  In Open chain alkanes the number of carbon atoms may be one (CH4)  two (C2H6) etc.In cycloalkane ,the total  number of carbon atoms may be either equal to 3 or more than 3 And cannot be less than three.
  Open chain alkanes have 2 hydrogen atoms more than the respective cycloalkanes. 
Cycloalkanes have two hydrogen atoms less than the parent open-chain alkanes due to the formation of an extra bond for ring closure.
General differences between open chain alkanes and cycloalkanes

Structure of open chain ( Aliphatic )Alkanes and Cycloalkanes:

             Carbons in aliphatic alkanes and cycloalkanes are sp3 hybridized.

sp3 Hybridization

             The hybridization in which one s-orbital and three p-orbitals are mixed to form four identical hybrid orbitals is called sp3 hybridization. The four sp3 hybrid orbitals degenerate (have equal energy) and they are directed in space to four corners of tetrahedron having an angle of 109.5o. Each sp3 hybrid orbital has 25% s-character and 75% p-character.

  Structure of Methane (CH4):

In methane and all other alkanes, carbon atoms are sp3 hybridized. The electronic configuration of carbon is 1s2, 2s2, 2p2. Its valence shell has 2 electrons in s-orbital and one electron in each of two p-orbitals while the third p-orbital is lying vacant.

By absorption of energy one electron of 2s promotes to vacant p-orbital and the atom becomes in an excited state, which is not a stable state. Thus to gain a state of stability mixing of atomic orbitals take place and as a result, four hybrid orbitals of equal energy and similar shape are produced. Each hybrid orbital is oriented towards the corner of a tetrahedron.  Each hybrid orbital has a single electron in it. So carbon is tetravalent and not divalent.

Half-filled s-orbitals of four hydrogen atoms overlap with half-filled sp3 hybrid orbitals of carbon forming four C-H sigma bonds. These bonds are formed due to sp3-s overlapping. As overlapping has taken place on the bond axis so sigma bonds are formed.

The shape of the methane molecule is tetrahedral. The bond angle between any two C-H bonds is 109.5o.

CH4

Structure of Ethane (C2H6):

 Both carbons in ethane are sp3 hybridized. Half-filled s-orbitals of six hydrogen atoms overlap with half-filled sp3 hybrid orbitals of two carbons forming six C-H sigma bonds. These bonds are formed due to sp3-s overlapping. One C – C sigma bond is formed due to the overlapping half-filled sp3 orbitals of the two carbons. The C – C sigma bond is formed due to sp3-sp3 overlapping. As overlapping has taken place on the bond axis so sigma bonds are formed. The bond angle between any two C-H bonds is 109.5o.

                                Ethane             

Structure of Cyclopropane (C3H6):

 All the three carbons in cyclopropane are sp3 hybridized. The sp3-sp3 overlapping of C – C sigma bond in cyclopropane is less as compared to that in aliphatic alkanes. This is due to the fact that in cyclopropane C – C – C bond angle is 60o instead of 109.5o. Hence orbitals do not overlap exactly along their axis. The small bond angles of cyclopropane and other cycloalkanes indicate that the overlap of sp3 orbitals of carbon in cycloalkanes is less than that of sp3 orbitals of carbon in aliphatic alkanes. The bonds are banana-shaped.

In cyclopropane, the three carbon atoms are present at the three corners of an equilateral triangle, which result in  C–C–C bond angles to be 60o.

The decrease (deviation) in bond angle from a normal tetrahedral bond angle (109.5o) due to less overlapping of sp3 hybrid orbitals is called Bond angle Strain.”

                               Cyclopropane

The more is the departure (deviation) from the normal tetrahedral angle (109.5o), the greater is strain. The greater the amount of strain, the greater would be the instability in the molecule. In other words, more is the bond angle strain, less is the overlapping of orbitals and less is the stability.

In cyclopropane C – C – C bond angle departs from an ideal tetrahedral bond angle by 109.5o – 60o = 49.5o

In cyclobutane C – C – C bond angle departs from an ideal tetrahedral bond angle by 109.5o – 88o

= 49.5o = 21.5o.

In cyclopentane C – C – C bond angle departs from an ideal tetrahedral bond angle by 109.5o – 108o = 1.5o.

So bond angle strain order Cyclopropane > cyclobutane > cyclopentane

General Physical Properties of open chain alkanes:

  1. At room temperature and pressure 1st four alkanes (i.e. from C1 to C4) are colourless and odourless gases. Alkanes from C5 to C17 are colourless and odourless liquids while alkanes from C18 and onward are colourless and odourless wax-like solids.
  2. The melting and boiling points of alkanes are lower than alcohols and other organic compounds of comparable molecular masses.
  3. The melting and boiling points of alkanes show a gradual increase with increasing molecular masses. However, the higher alkanes show no marked change in melting and boiling point.
  4. More is the number of branches in alkanes lower will be the boiling points.
  5. Alkanes are non-polar so are soluble in non-polar solvents like ether, acetone, benzene, carbon tetrachloride etc.
  6. In specific gravity, the viscosity of alkanes increases with increasing molecular masses.

General Physical Properties of Cycloalkanes:

  1. Cyclic alkanes start from three carbons i.e. cyclopropane.
  2. The first two members i.e. cyclopropane and cyclobutane are gases at R.T.P and the rest are liquid.
  3. They are insoluble in water but soluble in benzene, carbon tetrachloride and ether.
  4. Their melting and boiling points show a gradual increase with increasing molecular masses.
  5. Their melting and boiling points are higher than corresponding aliphatic alkanes.
  6. Cyclopropane and cyclobutane are the least stable while cyclohexane is the most stable.
  7. Their general formula is CnH2n.

Reactivity of open chain Alkanes:

Alkanes are less reactive i.e. they are comparatively inert and are hence called paraffins. It is from the Latin word parum meaning little and affin meaning affinity.

The low reactivity of alkanes is due to

(i) The reason is that they are saturated compounds having strong covalent bonds which are difficult to break.

(ii) The reason that they are non-polar so electrophiles, as well as nucleophiles, cannot attack them.

Due to these reasons, alkanes are inert to acids, bases, oxidizing and reducing agents under ordinary conditions. However, at elevated temperatures, they react with these reagents.

Alkanes being saturated compounds mostly show substitution reactions. Their oxidation is very difficult thus even strong oxidizing agents like KMnO4, K2Cr2O7, etc cannot oxidize them.

Reactivity of Cycloalkanes:

The strength of the bond and thus the stability depends on the extent of overlap of orbitals. In the case of Cyclopropane and cyclobutane, the extent of overlap is less due to greater angle strains and hence they are unstable. 

Cyclopropane undergoes ring-opening reactions with H2/Ni and HBr to give open-chain addition products. While cyclobutane, having less angle strain than that of cyclopropane, is more stable and hence it undergoes ring-opening reactions with H2/Ni and HBr to give open-chain addition products only under severe conditions.


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