Keto-Enol Tautomerism: Key Points

The reason for the equilibrium lying to the left is due to bond energies. The keto form has a C–H, C–C, and C=O bond whereas the enol has a C=C, C–O an O–H bond. The sum of the first three is about 359 kcal/mol (1500 kJ/mol) and the second three is 347 kcal/mol (1452 kJ/mol). The keto form is therefore more thermodynamically stable by 12 kcal/mol (48 kJ/mol).

Although the keto form is most stable for aldehydes and ketones in most situations, there are several factors that will shift the equilibrium toward the enol form.  The same factors that stabilize alkenes or alcohols will also stabilize the enol form. There are two strong factors and three subtle factors.

1. Aromaticity.  Phenols can theoretically exist in their keto forms, but the enol form is greatly favored due to aromatic stabilization.

2. Hydrogen Bonding. Nearby hydrogen bond acceptors stabilize the enol form. When a Lewis basic group is nearby, the enol form is stabilized by internal hydrogen bonding.

Here are three more subtle effects in keto-enol tautomerism:

3. Solvent. Solvent can also play an important role in the relative stability of the enol form. For example, in benzene, the enol form of 2,4-pentanedione predominates in a 94:6 ratio over the keto form, whereas the numbers almost reverse completely in water. What’s going on? In a polar protic solvent like water, the lone pairs will be involved in hydrogen bonding with the solvent, making them less available to hydrogen bond with the enol form.

 4. Conjugation . π systems are a little like Cheerios in milk: given the choice, they want to connect together than hang out in isolation.  So in the molecule depicted, the more favorable tautomer will be the one on the left, where the double bond is a connected by conjugation to the phenyl.

 5. Substitution. In the absence of steric factors, increasing substitution at carbon will stabilize the enol form. Enols are alkenes too – so any factors that stabilize alkenes, will stabilize enols as well. All else being equal, double bonds increase in thermodynamic stability as substitution is increased. So in the above example, the enol on the left should be the more stable one. As you might suspect, “all things being equal” sounds like a big caveat. It is – all else is rarely equal. But that’s a topic for another day – or, more likely, another course.

 

Sources: “March’s Advanced Organic Chemistry”, “Solvents and solvent effects in Organic Chemistry”, by Christian Riechart. EDIT: Commenter Natalia helpfully points out that Carey & Sundberg A is a great resource for this topic (section 7.3 in my 4th edition) and she is right.

 One really interesting observation is that when you dissolve acetone in D2O, you slowly get incorporation of deuterium at the alpha carbon. This is the enol tautomer at work – it reacts with a proton/deuteron source at the alpha carbon and regenerates the ketone.

Under normal conditions, this is a pretty slow reaction. But when you add a bit of acid, suddenly the rate of deuterium incorporation drastically increases. What’s going on here?

Let’s call the rate for conversion of ketone into enol K1 and the rate for conversion of enol into ketone K2.  The cool thing is, acid speeds up BOTH reactions for keto-enol tautomerism – both the forward and the reverse reaction. Here’s how it works.

1)  For K1 (keto to enol), acid protonates the carbonyl, making the carbonyl carbon more electrophilic. The more electrophilic the carbonyl  the stronger an acid it becomes at the alpha carbon (remember, trifluoroacetone is more acidic than acetone). So deprotonation at the alpha carbon becomes much easier in the presence of acid, which results in the enol form.

2) For K2, acid also speeds up the conversion of the enol to the keto form. The enol form is nucleophilic at the alpha carbon, and reacts with the strongest electrophile around. An acid – say, D2SO4  – is a much, much stronger electrophile than D2O, so this is going to increase the rate of the enol to ketone conversion.

naming bridgehead carbons

Nomenclature of spiro compounds


            Compounds in which one carbon atom is common to two different rings are called spiro compounds. The IUPAC name for a spiro compound
begins with the word spiro followed by square brackets containing the number of carbon molecules, in increasing order, in each ring connected to the common carbon atom and then by the name of the parent hydrocarbon corresponding to the total number of the carbon atoms in the two rings. The position of substituents are indicated by numbers ; the numbering beginning with the carbon atom adjacent to the spiro carbon and proceeding first around the smaller ring and then to the spiro atom and finally around the larger ring For example,

then  to the spiro atom and finally around the larger ring.

 For example,

 

IUPAC NOMENCLATURE OF ORGANIC COMPOUNDS: INTRODUCTION

 In the earlier days, the conventional names for organic compounds were mainly derived from the source of occurrence. However organic chemists realized the need for a systematic naming for organic compounds since a large number of organic compounds are synthesized in due course. This leads to setting up a system of nomenclature by  "International Union of Pure and Applied Chemistry, IUPAC" . 

The IUPAC system of nomenclature is a set of logical rules framed which are mainly aimed at giving an unambiguous name to an organic compound. By using this system, it is possible to give a systematic name to an organic compound just by looking at its structure and it is also possible to write the structure of organic compound by following the IUPAC name for that compound.

On this page, I have given a logical introduction to IUPAC nomenclature. A concise and unified approach is followed to help in giving IUPAC names to almost all types of compounds. This is not an exhaustive reference to IUPAC nomenclature. However this is more than suffice to all the students at various levels of their learning curve.

SYSTEMATIC IUPAC NAME

The systematic IUPAC name of an organic compound consists of four parts.

1.      Root word

2.      Suffix(es) and

3.      Prefix(es)

4.      infix

The suffix is again divided into primary and secondary.

The complete systematic IUPAC name can be represented as:

* The root word and 1^osuffix together is known as base name. 

* The Prefix(es), infix and 2^o suffix may or may not be required always. 

1) Root word: It indicates the number of carbon atoms in the longest possible continuous carbon chain also known as parent chain chosen by a set of rules. The root words used for different length of carbon chain (upto 20) are shown below.

Number of carbon atoms in the parent chain	Root word
1	Meth
2	Eth
3	Prop
4	But
5	Pent
6	Hex
7	Hept
8	Oct
9	Non
10	Dec
11	Undec
12	Dodec
13	Tridec
14	Tetradec
15	Pentadec
16	Hexadec
17	Heptadec
18	Octadec
19	Nonadec
20	Icos

2) Suffix: It is again divided into two types.

                                i.            Primary suffix and

                              ii.            Secondary suffix

i) Primary suffix: It is used to indicate the degree of saturation or unsaturation in the main chain. It is added immediately after the root word. 

Type of carbon chain	Primary suffix
Saturated (all C-C bonds)	-ane
Unsaturated: one C=C	-ene
Unsaturated: two C=C	-diene
Unsaturated: one C≡C	-yne
Unsaturated: two C≡C	-diyne
Unsaturated: one C=C & one C≡C	-enyne

ii) Secondary suffix: It is used to indicate the main functional group in the organic compound and is added immediately after the 1^o suffix.

Note: If there are two or more functional groups in a compound, the functional group with higher priority is to be selected as main functional group, which must be indicated by a secondary suffix. The remaining functional groups with lower priority are treated as substituents and are indicated by prefixes.

The suffixes as well as prefixes used for some important functional groups are shown in the following table in the decreasing order of their priority. 

Also note that different suffix is used when carbon atom of the functional group is not part of the main chain.

Name of Functional group	Representation	Suffix When carbon of the functional group is part of the parent chain	Suffix When carbon of the functional group is NOT part of the parent chain	Prefix
carboxylic acid	-COOH	-oic acid	-carboxylic acid	carboxy-
Acid anhydride		-oic anyhydride	-carboxylic anhydride	-
Ester	-COOR	alkyl -oate	alkyl -carboxylate	alkoxycarbonyl-
Acid halide	-COX	-oyl halide	-carbonyl halide	halocarbonyl-
Acid amide	-CONH2	-amide	-carboxamide	carbamoyl-
Nitrile	-CN	-nitrile	-carbonitrile	cyano-
Aldehyde	-CHO	-al	-carbaldehyde	oxo-
Ketone	-CO-	-one	-	oxo-
Alcohol	-OH	-ol	-	hydroxy
Thiol	-SH	-thiol	-	mercapto
Amine	-NH2	-amine	-	amino-
Imine	=NH	-imine	-	imino-
Alkene	C=C	-ene	-	-
Alkyne	C≡C	-yne	-	-

 Note: This is not the complete reference.

3) Prefix: The prefix is used to indicate the side chains, substituents and low priority functional groups (which are considered as substituents). The prefix may be added immediately before the root word or before the infix.

The prefixes used for some common side chains and substituents are shown below. (the prefixes for functional groups are already given)

Side chain or Substituent	Prefix
-CH3	methyl-
-CH2CH3 (or) -C2H5	ethyl-
-CH2CH2CH3	propyl-
	isopropyl-
-CH2CH2CH2CH3	butyl
	sec-butyl  (or)  (1-methyl)propyl
	isobutyl (or) (2-methyl)propyl
	tert-butyl (or) (1,1-dimethyl)ethyl
-X	halo-
-OR	alkoxy-
-NO2	-nitro

 Remember that the alkyl groups along with halo, nitro and alkoxy have the same preference. They have lower priority than double and triple bonds.

3) Infix: The infixes like cyclo, spiro, bicyclo are added between the prefix(es) and root word to indicate the nature of parent chain. 

* The "Cyclo" infix is used to indicate the cyclic nature of the parent chain.

* The "Spiro" infix is used to indicate the spiro compound.

* The "Bicyclo" infix is used to indicate the bicyclic nature of the parent chain.

The infixes are some times called as primary prefixes.

STEPS INVOLVED IN GIVING IUPAC NAME

1) The first step in giving IUPAC name to an organic compound is to select the parent chain and assign a root word. 

2) Next, the appropriate primary prefix(es) must be added to the root word to indicate the saturation or unsaturation.

3) If the molecule contains functional group or groups, a secondary suffix must be added to indicate the main functional group. This is optional and not necessary if the molecule contains no functional group.

4) Prefix the root word with the infix "cyclo" if the parent chain is cyclic; or with the infix "spiro" if it is a spiro compound; or with the infix "bicyclo" if the compound is bicyclic.

5) Finally add  prefix(es) to the name if there are side chains or substituents on the parent chain.

E.g. The IUPAC name of the following compound is arrived in steps mentioned below.

Step-1	How many carbons are there in the parent chain?	4	Root word = "but"
Step-2	Saturated or Unsaturated?	Saturated	1osuffix = "ane"
Step-3	Is there any functional group?	Yes. There is an alcohol group on 2nd carbon.	2osuffix = "2-ol"
Step-4	Are there any side chains or substituents?	Yes. There is a methyl group on 3rd carbon.	2oprefix = "3-methyl"

Now add them to makeup the IUPAC name of the compound. 

You will learn how to select a parent chain?; how to number the carbon atoms and give the locants to the functional groups, side chains ? etc., in the following section.

IUPAC RULES

The following rules are helpful in assigning the systematic IUPAC name of an organic compound.

1) The selection of parent chain: The first step in naming an organic compound is to select the parent chain and give the root word depending on the number of carbons in it.

The longest continuous carbon chain containing as many functional groups, double bonds, triple bonds, side chains and substituents as possible is to be selected as parent chain.

Illustrations:

i) In the following molecule, the longest chain has 6 carbons. Hence the root word is "hex-". Note that the parent chain may not be straight.

ii) The root word for the following molecule is "hept-" since the longest chain contains 7 carbons. 

Do not come under the impression that the ethyl groups (-C₂H₅) are side chains and the longest chain contains 5 carbons. 

The shaded part shows the longest chain that contains 7 carbons. Also look at the alternate way of writing this molecule in which the ethyl groups are expanded to -CH₂CH₃.

iii) In the following molecule, there are three chains of equal length (7 carbons).

However the chain with more number of substituents (that with 3 substituents as shown in the following diagram) is to be taken as the parent chain

iv) The double bonds and triple bonds have more priority than the alkyl side chains and some other substituents like halo, nitro, alkoxy etc. Hence, whenever there are two or more chains with equal number of carbons, the chain that contains double or triple bond is to be selected as the parent chain irrespective of other chain containing more number of substituents. 

There are two chains with 6 carbons. But the chain with the a double bond as shown in the diagram (II) is to be selected as the parent chain.

Note: The double bond has more priority than the triple bond.

v) However, the longest chain must be selected as parent chain irrespective of whether it contains multiple bonds or not.

E.g. In the following molecule, the longest chain (shaded) contains no double bond. It is to be selected as parent chain since it contains more carbons (7) than that containing double bond (only 6 carbons).

vi) The chain with main functional group must be selected as parent chain even though it contains less number of carbons than any other chain without the main functional group.

The functional group overrides all of above rules since it has more priority than the double bonds, triple bonds, side chains and other substituents.

E.g. The chain (shaded) with 6 carbons that includes the -OH functional group is to be selected as parent chain irrespective of presence of another chain with 7 carbons that contains no functional group.

There are other situations which will decide the parent chain. These will be dealt at appropriate sections.

2) Numbering the parent chain:  

i) The positions of double bonds or triple bonds or substituents or side chains or functional groups on the parent chain are to be indicated by appropriate numbers (or locants). The locants are assigned to them by numbering carbon atoms in the parent chain. 

Even though two different series of locants are possible by numbering the carbon chain from either sides, the correct series is chosen by following the rule of first point of difference as stated below.

The rule of first point of difference: When series of locants containing the same number of terms are compared term by term, that series which contains the lowest number on the occasion of the first difference is preferred.  

For example, in the following molecule, the numbering can be done from either side of the chain to get two sets of locants. However the 2,7,9 is chosen since it has lowest number i.e., 2 on the first occasion of difference when compared with the other set: 3,4,9.

Actually the so called “Least Sum Rule” is the special case of above “Rule of First point of Difference”. Though looking simple, the least sum rule is valid only to chains with two substituents, a special case. However use of Least sum rule is not advisable when there are more than two substituents since it may violate the actual rule of first point of difference. 

Therefore, while deciding the positions, we should always use "the rule of first point of difference" only.

ii) If two or more side chains are at equivalent positions, the one to be assigned the lower number is that cited first in the name.

In case of simple radicals, the group to be cited first in the name is decided by the alphabetical order of the first letter in case of simple radicals. While choosing the alphabetical order, the prefixes like di, tri, tetra must not be taken into account.

In the following molecule, 4-ethyl-5-methyloctane, both methyl and ethyl groups are at equivalent positions. However the ethyl group comes first in the alphabetical order. Therefore it is to be written first in the name and to be given the lowest number.

Note: The groups: sec-butyl and tert-butyl are alphabetized under "b". However the Isobutyl and Isopropyl groups are alphabetized under "i" and not under "b" or "p".

iii) However, if two or more groups are not at equivalent positions, the group that comes first alphabetically may not get the least number. 

E.g. In the following molecule, 5-ethyl-2-methylheptane, the methyl and ethyl groups are not at equivalent positions. The methyl group is given the least number according to the rule of first point of difference.

But note that the ethyl group is written first in the name.

iv) The multiple bonds (double or triple bonds) have higher priority over alkyl or halo or nitro or alkoxy groups, and hence should be given lower numbers.

E.g. In the following hydrocarbon, 6-methylhept-3-ene, the double bond is given the lower number and is indicated by the primary suffix 3-ene. The position of methyl group is indicated by locant, 6.

v) The double bond is preferred over the triple bond since it is to be cited first in the name.

Therefore the double bond is to be given the lower number whenever both double bond and triple bond are at equivalent positions on the parent chain.

E.g. In the following hydrocarbon, hept-2-en-5-yne, both the double and triple bonds are at equivalent positions. But the position of double bond is shown by 2-ene. The counting of carbons is done from the left hand side of the molecule.

vi) However, if the double and triple bonds are not at equivalent positions, then the positions are decided by the rule of first point of difference.

E.g. In the following hydrocarbon, hept-4-en-2-yne,  the double and triple bonds are not at equivalent positions. The triple bond gets the lower number

Again note that the 4-ene is written first.

vii) Nevertheless, the main functional group must be given the least number even though it violates the rule of first point of difference. It has more priority over multiple bonds also.

For example, in the following organic molecule, 6-methyloct-7-en-4-ol, the -OH group gets lower number (i.e., 4) by numbering the carbons from right to left.

v) If the side chains themselves contain terms like di, tri, tetra etc., the multiplying prefixes like bis, tris, tetrakis etc., should be used.

E.g. The two 1,2-dimethylpropyl groups are indicated by the prefix "bis" as shown below

vi) If two or more side chains of different nature are present, they are cited in alphabetical order. 

* In case of simple radicals, they are alphabetized based on the first letter in the name of simple radical without multiplying prefixes.

E.g. In the following molecule, the ethyl group is written first since the letter 'e' precedes the letter 'm' of methyl in the alphabetical order. We should not compare 'e' in the word 'ethyl' and 'd' in the word 'dimethyl'

* However the name of a complex radical is considered to begin with the first letter of its complete name.

E.g. In the following case, “dimethylbutyl” is considered as a complete single substituent and is alphabetized under "d".