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Why are 7-membered rings less likely to form than 5- and 6- membered rings?
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Muneer Ahmed
Why are 7-membered rings less likely to form than 5- and 6- membered rings?
There are two factors that need to be considered to answer your question - entropy and enthalpy.
Entropy favors the formation of smaller rings. If we consider a hydrocarbon chain, the number of possible conformations increases dramatically as we go from 3 carbons to, for example, 10 carbons. Only a few of these conformations will be correctly arranged so that the two ends of the chain are close to each other, which is necessary if we are to form a bond. So, entropically speaking, it will be much more probable to form a 3-membered ring than a 10-membered ring. This is shown in the following graph where 3-, 4- and 5-membered ring formation is entropically most favored.
Note that a less negative $\Delta S$ favors the reaction since $\Delta G=\Delta H - T \Delta S$.
Enthalpy comes into play when we consider the strain in the transition state leading to the possible products. Here is a chart showing the relative ring strain as a function of ring size.
As expected, strain is the highest in 3- and 4-membered rings.
If we overlay (add) these entropic and enthalpic effects we would get a graph similar to the one pictured below.
In general, we find that 5-membered ring formation is usually the favored pathway. This web article (J. Am. Chem. Soc.1984, 106, 1051-1056.) is a nice review of this topic that may be useful for a more in-depth reading on the subject. The above web article is also the source of the above three images.
There are two factors that need to be considered to answer your question - entropy and enthalpy.
Entropy favors the formation of smaller rings. If we consider a hydrocarbon chain, the number of possible conformations increases dramatically as we go from 3 carbons to, for example, 10 carbons. Only a few of these conformations will be correctly arranged so that the two ends of the chain are close to each other, which is necessary if we are to form a bond. So, entropically speaking, it will be much more probable to form a 3-membered ring than a 10-membered ring. This is shown in the following graph where 3-, 4- and 5-membered ring formation is entropically most favored.
Note that a less negative $\Delta S$ favors the reaction since $\Delta G=\Delta H - T \Delta S$.
Enthalpy comes into play when we consider the strain in the transition state leading to the possible products. Here is a chart showing the relative ring strain as a function of ring size.
As expected, strain is the highest in 3- and 4-membered rings.
If we overlay (add) these entropic and enthalpic effects we would get a graph similar to the one pictured below.
In general, we find that 5-membered ring formation is usually the favored pathway. This web article (J. Am. Chem. Soc.1984, 106, 1051-1056.) is a nice review of this topic that may be useful for a more in-depth reading on the subject. The above web article is also the source of the above three images.
There are two factors that need to be considered to answer your question - entropy and enthalpy.
Entropy favors the formation of smaller rings. If we consider a hydrocarbon chain, the number of possible conformations increases dramatically as we go from 3 carbons to, for example, 10 carbons. Only a few of these conformations will be correctly arranged so that the two ends of the chain are close to each other, which is necessary if we are to form a bond. So, entropically speaking, it will be much more probable to form a 3-membered ring than a 10-membered ring. This is shown in the following graph where 3-, 4- and 5-membered ring formation is entropically most favored.
Note that a less negative $\Delta S$ favors the reaction since $\Delta G=\Delta H - T \Delta S$.
Enthalpy comes into play when we consider the strain in the transition state leading to the possible products. Here is a chart showing the relative ring strain as a function of ring size.
As expected, strain is the highest in 3- and 4-membered rings.
If we overlay (add) these entropic and enthalpic effects we would get a graph similar to the one pictured below.
In general, we find that 5-membered ring formation is usually the favored pathway. This web article (J. Am. Chem. Soc. 1984, 106, 1051-1056.) is a nice review of this topic that may be useful for a more in-depth reading on the subject. The above web article is also the source of the above three images.
There are two factors that need to be considered to answer your question - entropy and enthalpy.
Entropy favors the formation of smaller rings. If we consider a hydrocarbon chain, the number of possible conformations increases dramatically as we go from 3 carbons to, for example, 10 carbons. Only a few of these conformations will be correctly arranged so that the two ends of the chain are close to each other, which is necessary if we are to form a bond. So, entropically speaking, it will be much more probable to form a 3-membered ring than a 10-membered ring. This is shown in the following graph where 3-, 4- and 5-membered ring formation is entropically most favored.
Note that a less negative $\Delta S$ favors the reaction since $\Delta G=\Delta H - T \Delta S$.
Enthalpy comes into play when we consider the strain in the transition state leading to the possible products. Here is a chart showing the relative ring strain as a function of ring size.
As expected, strain is the highest in 3- and 4-membered rings.
If we overlay (add) these entropic and enthalpic effects we would get a graph similar to the one pictured below.
In general, we find that 5-membered ring formation is usually the favored pathway. This web article (J. Am. Chem. Soc. 1984, 106, 1051-1056.) is a nice review of this topic that may be useful for a more in-depth reading on the subject. The above web article is also the source of the above three images.
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