Conformations of Polycyclic Molecules


After completing this section, you should be able to draw the structures and construct molecular models of cis– and trans-decalin and of norbornane.


Make certain that you can define, and use in context, the key terms below.

  • bridgehead carbon atom
  • polycyclic molecule

Study Notes

A bridgehead carbon atom is a carbon atom which is shared by at least two rings. The hydrogen atom which is attached to a bridgehead carbon may be referred to as a bridgehead hydrogen.

Note that bicyclo[2.2.1]heptane is the systematic name of norborane. You need not be concerned over the IUPAC name of norbornane. The nomenclature of compounds of this type is beyond the scope of this course.

Bicyclic Ring Systems

A bridged bicycloalkane is a bicycloalkane whose molecule has two carbon atoms shared by all three rings identifiable in the molecule.  The two carbon atoms shared by the three rings are called bridgehead carbon atoms.  A bond or a chain of bonds connecting the bridgehead carbon atoms is called a bridge.

Eg. 1:  Decalin

Eg. 2:  bicyclo[2.2.1]heptane

If in a bridged bicycloalkane, the bridgehead carbons are directly bonded to each other, the compound is called a fused bicycloalkane. In other words, in a fused bicycloalkane, the number of carbon atoms in one of the three bridges is zero. eg: decalin

see also spirobicycloalkane

Decalins can come in two diastereomers, the trans– or cis– diastereomer.  The trans-diastereomer is a rigid structure which cannot undergo a ring flip.  The cis-diastereomer is mobile and can ring flip to allow substituents to sit in the equatorial position.


Steroids include such well known compounds as cholesterol, sex hormones, birth control pills, cortisone, and anabolic steroids.



The best known and most abundant steroid in the body is cholesterol. Cholesterol is formed in brain tissue, nerve tissue, and the blood stream. It is the major compound found in gallstones and bile salts. Cholesterol also contributes to the formation of deposits on the inner walls of blood vessels. These deposits harden and obstruct the flow of blood. This condition, known as atherosclerosis, results in various heart diseases, strokes, and high blood pressure.

Much research is currently underway to determine if a correlation exists between cholesterol levels in the blood and diet. Not only does cholesterol come from the diet, but cholesterol is synthesized in the body from carbohydrates and proteins as well as fat. Therefore, the elimination of cholesterol rich foods from the diet does not necessarily lower blood cholesterol levels. Some studies have found that if certain unsaturated fats and oils are substituted for saturated fats, the blood cholesterol level decreases. The research is incomplete on this problem.


Structures of Sex Hormones

Sex hormones are also steroids. The primary male hormone, testosterone, is responsible for the development of secondary sex characteristics. Two female sex hormones, progesterone and estrogen or estradiol control the ovulation cycle. Notice that the male and female hormones have only slight differences in structures, but yet have very different physiological effects.

Testosterone promotes the normal development of male genital organs ans is synthesized from cholesterol in the testes. It also promotes secondary male sexual characteristics such as deep voice, facial and body hair.

Estrogen, along with progesterone regulates changes occurring in the uterus and ovaries known as the menstrual cycle. For more details see Birth Control. Estrogen is synthesized from testosterone by making the first ring aromatic which results in mole double bonds, the loss of a methyl group and formation of an alcohol group.




Someone stated that trans-decalin is more stable than cis-decalin. Explain why this is incorrect.



  • John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, “You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format.”