Posts Tagged "organic"

Chirality and Assigning Stereochemistry to Molecules

Posted on August 11th, 2015

One of the most important skills to master in organic chemistry is the ability to assign stereochemistry.  We at StudyOrgo have devised clear cut explanations of these difficult concepts for students to maximize their time studying and learn difficult concepts quickly and easily. Sign up with today for all of your organic chemistry studying needs!

Chirality is an important aspect of life.  This is so because many of the basic molecules used in living cells, in particular amino acids that form enzymes, are also chiral. Chirality imparts asymmetry on our molecules, allowing them the ability to recognize “handedness” and further add to the complexity and specificity of reactions. As organic chemists, we must pay constant attention to the chirality of molecules both before and after reactions, less the compounds lose their biological or chemical activity.

Chirality is defined as any object in which the mirror images are not superimposable. A good example is your hands; they are mirror images but not superimposable. Translating this to organic molecules, a stereocenter is a carbon center with 4 unique substituents that are arranged such that the mirror image is not superimposable. Thus, they “look” like to different molecules although they have the same substituents. If we alter the arrangement of the substituents, we can always come up with 2 arrangements for each substituent, R or S configuration.  Thus, each stereocenter must have 2 stereoisomers.

chiral 1

In order to determine whether the sterecenter is the the R or S configuration, there are a series of steps to follow.

  1. Identify the stereocenter as 4 unique substituents attached to the chiral center
  2. Assign priority based on atom atomic number, highest (1) to lowest (4) weight.
  3. If two atoms are same, move to next bond to find first point of difference
  4. Rotate the molecule so that Priority 4 atom is in the hashed wedge position.
  5. Determine the Priority sequence 1-2-3 rotates to the left (S) or the right (R).

chiral 2

Lastly, an important concept to keep in mind is that as molecules become more complex, they also can acquire more stereocenters.  Keeping in mind that each stereocenter can produce 2 stereoisomers, we describe possible stereoisomerism using the 2n rule. Let’s examine a molecule with 2 stereocenters, following the 2n rule that gives us 22=4 stereocenters.  The possible combinations are listed below.

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We now introduce the last concept to stereochemistry which is the difference between enantiomers and diastereomers.  Enantiomers are molecules with exactly opposite stereoisomers.  For example, the enantiomer of the molecule with stereochemistry R,R would be S,S.  The relationship between molecule R,R and R,S is what is described as diastereomers, which differ in some but not all stereocenters.

Let’s consider the biologically active form of testosterone, 5-DHT which is shown below.  We indicate that it has 7 stereocenters in the molecule.  Applying the 2n rule, we calculate 128 possible stereoisomer combinations.  That concludes that while testosterone has 1 enantiomer, it has 126 diastereomers and remember…only 5-DHT works on our bodies!

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Organic Chemistry of Lipids

Posted on May 11th, 2015

Lipids are a fundamental part of biochemistry and draw many analogies to reactions with alkenes and alkanes.  In this article, we will review some basics of lipids and their organic chemistry implications.  Many of the reactions with lipids are simple hydrocarbon reaction mechanisms covered in Organic Chemistry 1.  With over 175 reaction mechanisms, StudyOrgo is devoted to making organic chemistry reaction mechanisms easy to learn and points out common places where errors are made.  Sign up with today to get more in-depth coverage of common reactions for your next exam!

Lipids are any complex chain of hydrocarbons that may or may not contain additional functional groups. Some common classes of lipids are fatty acids and triglycerides, waxes, terpenes and steroids; examples of which are shown in Figure 1. 

Lipid Figure 1

Most lipids usually consist of a long hydrophilic tail coupled to a polar head group that undergoes reactions to elongate or shorted the hydrophobic tail.  We will cover some common organic chemistry reactions with each of these classes of lipids.

Complex Lipids – Fatty acids and Triglycerides

Complex lipids are capable of undergoing hydrolysis reactions.  The basic unit of a complex lipids are fatty acids, which are made up of a hydrophobic tail coupled to a carboxylic acid head group (Figure 2).  The length of the carbon chain denotes the lipid name.  In addition, the number of double bonds, or degree of unsaturation, also influences the name.  In biology, naturally occurring double bonds will always be found in the cis- configuration.

Lipid Figure 2

Hydrogenation of alkene groups to alkanes occurs at high temperatures, such as in deep fryers for cooking, and this side reaction leads to the isomerization of the double bond to the trans- conformation, hence trans-fatty acids.  These lipids are toxic in high quantities because they are unable to be metabolized by the cell, thus accumulating and undergoing oxidation reactions over time and theorized to promote inflammation and metabolic diseases.  Coupling of a fatty acids to the alcohol groups of glycerol forms the complex lipid, triglyceride (Figure 2).  This reaction occurs enzymatically in the cell but closely resembles the mechanism found in acid-catalyzed Fisher Esterification.

Lipid Figure 3

Simple Lipids – Sterols

Steroids are an integral part of cellular biology but are classified as simple lipids because they cannot be hydrolyzed (i.e. they do not have reactive carbonyl or carboxyl groups).  A lipid molecule having a tetracyclic shape in the arrangement similar to cholesterol is classified as a sterol.  Cholesterol serves as the building block of all steroids and substitution, addition and elimination of functional groups derives the variety of steroids found in the body. Cholesterol is synthesized from the 5 carbon lipid molecule isopentenyl phosphate.  Condensation of isopentenyl phosphate forms geranyl phosphate (10C) and farnesyl phosphate (15C) in a reaction mechanism that involves allylic carbocation and tertiary carbocation intermediates (Figure 3).  Condensation of two farnesyl phosphate molecules forms the 30 carbon intermediate squalene, which in several steps is converted to cholesterol.  Two important examples of steroids are the sex hormones found in humans, estradiol and testosterone, which regulate a wide range of biological functions.

Lipid Figure 4