An isomer is a molecule that has the same molecular formula as another molecule, but
possesses different chemical properties. Another way to say this is that isomers contain the
same number of atoms of each element, but the arrangement of these atoms is different.
There are two kinds of isomerism; structural isomerism (wherein functional groups are
attached in different ways) and stereoisomerism. Stereoisomers are isomeric molecules
whose molecular formula is the same, but the 3-D orientations of their constituent atoms in
space are different. Stereoisomers are further divided into two types: enantiomers and
diastereomers.
Enantiomers have chiral carbon atoms. A chiral carbon is a carbon center that is bonded to
four different atoms or groups. The presence of a chiral carbon (in a molecule) is called
chirality. In order to become an enantiomer, two molecules should have different
configurations at every chiral carbon. For example, if a molecule has two chiral carbons and
another molecule has the same molecular formula with two chiral carbons, the two molecules
should be different at both chiral carbons, not just at one chiral carbon.
Epimers are stereoisomers that contain more than one chiral carbon but differ from each
other in the configuration at only one chiral carbon. Therefore, they are not mirror images of
each other. Epimers are a subclass of diastereomers. Therefore, epimers are also optical
isomers. This means epimers can rotate plane polarized light.
Formation of epimers is called epimerization. This epimerization process forms epimers by
changing a chiral carbon in a molecule which has several chiral carbons. Since the difference
in two epimers is in one chiral carbon, this carbon is called epimeric carbon.
Similarities Between Enantiomers and Epimers

Enantiomers and epimers are stereoisomers.

Both are types of optical isomers.

Both enantiomers and epimers show chirality.
Difference Between Enantiomers and Epimers
Definition

Enantiomers: Enantiomers are optical isomers that are non-superimposable mirror
images of each other.

Epimers: Epimers are stereoisomers that contain more than one chiral carbon but
differ from each other in the configuration at only one chiral carbon.
Chiral Carbons

Enantiomers: Enantiomers are different from each other at every chiral carbon.

Epimers: Epimers are different from each other at only one (or few) chiral carbons,
but not all.
Mirror Images

Enantiomers: Enantiomers are non-superimposable mirror images of each other.

Epimers: Epimers are not mirror images of each other.
Properties

Enantiomers: The physical and chemical properties of enantiomers are the same
except for the rotation of plane polarized light.

Epimers: The physical and chemical properties of epimers are different from each
other.
α (Alpha) – the name given to the configuration of a cyclic sugar where the oxygen on the
anomeric carbon is on the opposite face of the ring relative to the substituent on the other
carbon flanking the ring oxygen. Contrasted with beta (β) which is where the two substituents
are on the same faces of the ring.
Aldaric acid – a dicarboxylic acid derived from an aldose where both ends of the sugar have
been oxidized, e.g. from the action of nitric acid:
Aldose – a monosaccharide with an aldehyde (or masked aldehyde) functional group.
[“Masked aldehyde” here refers to a cyclic hemiacetal that is in equilibrium with an acyclic
aldehyde through ring-chain tautomerism]. Contrast aldoses with ketoses, which are
monosaccharides with a ketone (or masked ketone) group.
Anomer – the name given to two diastereomeric monosaccharides that are epimers at the
anomeric carbon. The two anomers are described with the terms α (“alpha”) and β (“beta”),
defined above.
Anomeric carbon – the carbon of a cyclic sugar bearing a hemiacetal or acetal (hemiketal or
ketal). This is C-1 in aldoses, and C-2 in the case of fructose.
β (beta) – the name given to the configuration of a cyclic sugar where the oxygen on the
anomeric carbon is on the same face of the ring as the substituent on the other carbon
flanking the ring oxygen. Contrast with alpha (α) where the two substituents are on the
opposite faces of the ring.
Carbohydrate – originally just referred to monosaccharides (such as glucose) with the
empirical formula Cn(H2O)n (i.e. “hydrates of carbon”). Now used as a more generic term
referring to mono-, oligo-, and polysaccharides as well as many of their derivatives.
D- (and L-) A monosaccharide is assigned to D- or L- according to the configuration at the
highest-numbered chiral center, i.e. the bottom-most chiral center in its Fischer projection. If
the hydroxyl group of this carbon is on the right, the sugar is assigned the prefix D-; if it is on
the left, it is assigned with the prefix L- . The enantiomer of a D-sugar is always an L-sugar.
The simplest sugar is glyceraldehyde, which has one chiral center and exists in two
enantiomeric forms, called D- and L- glyceraldehyde, respectively.
Disaccharide – a carbohydrate which can be hydrolyzed to give two monosaccharides. Some
prominent examples of disaccharides are lactose, sucrose, and maltose. The sugars of a
disaccharide are joined through a glycosidic bond.
Epimer – two sugars which differ in the configuration of a single chiral center. D-Glucose and
D-Mannose are epimers, as are D-Mannose and D-Talose. [Note: generally speaking, when
using the term “epimer”, one of the atoms on the chiral center is usually hydrogen]
Furanose – a five-membered cyclic hemiacetal. The name derives from furan, a five
membered cyclic ether. Furanoses are distinct from pyranoses, which is a six-membered cyclic
hemiacetal.
Glycoside – fancy name for an acetal (or ketal) formed from a sugar. Formally, a glycoside is
an acetal derived from condensation of the hemiacetal (or hemiketal) carbon of a sugar with
the hydroxy group of another compound.
Glycosidic bond – the bond that links the anomeric carbon of a sugar to another compound.
[For our purposes, this is through a C–O bond, but there are also such things as nitrogen,
sulfur, and selenium glycosides (among others)]
Haworth Projection – a graphical depiction of monosaccharides that clearly shows the
stereochemistry at each carbon (at the expense of accurately depicting its conformation).
Hexose – a sugar with six carbons, the most familiar example being glucose. A hexose that
bears an aldehyde (or masked aldehyde) is called an aldohexose; a hexose bearing a ketone
is called a ketohexose.
Ketose – a monosaccharide with a ketone (or masked ketone) functional group. Contrast with
aldoses.
L- (and D- ) When drawn in a Fischer projection,the highest-numbered chiral center in all Lsugars will be pointing to the left.
Monosaccharide – a sugar which cannot be hydrolyzed into simpler sugar units. Glucose,
ribose, mannose, and many other sugars (including deoxyribose) are monosaccharides.
Sucrose, by contrast, is a disaccharide which can be hydrolyzed into glucose and fructose.
Mutarotation – the change in optical rotation that occurs when a pure anomer equilibrates
to the different anomer through ring-chain tautomerism.
Oligosaccharide – a carbohydrate with a moderate (~ 3 to ~10) number of monosaccharide
units bound together by glycosidic bonds. There is no firmly defined boundary between
“oligosaccharide” and “polysaccharide”, but carbohydrate with more than 10 saccharide units
starts to get into polysaccharide territory.
Pentose – a five-carbon monosaccharide, such as ribose and deoxyribose. A pentose bearing
an aldehyde is called an aldopentose; a pentose bearing a ketone is called a ketopentose.
Polysaccharide – a carbohydrate composed of many monosaccharides linked together.
“Many” is somewhat arbitrary, but a carbohydrate with about ten or fewer monosaccharide
units is often called an oligosaccharide.
Pyranose – a six-membered cyclic hemiacetal. Derived from the name pyran, a six-membered
cyclic ether. Distinct from a furanose, which is a five-membered cyclic hemiacetal.
Reducing sugar – a mono- or disaccharide which gives a positive Tollens test. Practically, it
refers to any mono- or di-saccharide with a hemiacetal that can undergo ring-chain
tautomerism. Glycosides are not reducing sugars, since they are not in equilibrium with an
aldehyde or ketone.
Ring-Chain Tautomerism – the equilibrium between linear and cyclic forms that occurs in
aldoses and ketoses. Ring chain tautomerism is possible when the cyclic form has a
hemiacetal, but is not possible in glycosides (where the acetals and ketals are “locked”).
The "right hand" and "left hand" nomenclature is used to name the enantiomers of a chiral
compound. The stereocenters are labeled as R or S.
Consider the first picture: a curved arrow is drawn from the highest priority (1) substituent to
the lowest priority (4) substituent. If the arrow points in a counterclockwise direction (left
when leaving the 12 o' clock position), the configuration at stereocenter is considered S
("Sinister" → Latin= "left"). If, however, the arrow points clockwise,(Right when leaving the
12 o' clock position) then the stereocenter is labeled R ("Rectus" → Latin= "right"). The R or S
is then added as a prefix, in parenthesis, to the name of the enantiomer of interest.
Step 1:

Give each atom connected to the chiral center a priority based on its atomic number.
The higher the atomic number, the higher the priority.

So, based on this, bromine gets priority one, the oxygen gets priority two, the methyl
carbon is the third and the hydrogen is the lowest priority-four:
Step 2:

Draw an arrow starting from priority one and going to priority two and then to
priority 3:

If the arrow goes clockwise, like in this case, the absolute configuration is R.

As opposed to this, if the arrow goes counterclockwise then the absolute configuration
is S.

So, remember: Clockwise – R, Counterclockwise – S.