A functional group is the part of a molecule where most of its chemical reactions occur. It is the part that effectively determines a compound's chemical properties in addition to many of its physical properties.
general, non-aromatic hydrocarbon groups that would be obtained by removing a hydrogen atom from an alkane:
The symbol R
represent a generalized alkyl group. The group can be methyl, ethyl, propyl, or any of a multitude of others. You might think of R as representing the Rest of a molecule, which we aren't bothering to specify because it's not important
The R may be methyl, isopropyl, or ethyl:
aryl group (Ar)
an aromatic hydrocarbon group derived from an arene by removal of a hydrogen atom.
a general class of primary carboxylic acids may be represented by:
(general class of carboxylic acids)
The Ar may be C6H4CH3, C6H3(CH3)2, or C6H2(CH3)3:
aryl group derived from the removal of a proton from benzene
What is the name for the general fragment:
(RC=O) where C is connected to something else too
alkanoyl (alkyl + carbonyl)
aka: acyl group
Two common ones:
Major Nitrogen-, Oxygen-, and halogen-containing functional groups:
- carboxylic acid
- alkanol halide
- alkyl halide
Carboxylic acids can act as hydrogen bond donors and acceptors. Think about the locations for the hydrogen bonds between the two carboxylic acid molecules shown below.
In carboxylic acids, a positively polarized -OH hydrogen atom from one molecule is attracted to a negatively polarized oxygen atom of another molecule. This results in a weak force between the molecules that holds them together. These intermolecular attractions must be overcome for a molecule to break free from the liquid and enter the vapor state, so carboxylic acids typically have higher boiling points than alkanes or ketones, both of which cannot act as hydrogen donor/acceptors.
Other functional groups capable of H-bonding include alcohols ( ), amides ( ), amines (), and imines ( ).
Name the functional groups shown in the molecule below
Oxididation Levels and Functional Groups
Functional groups which have the same oxidation level can interconvert by substitution reactions. To go from one level to another, an oxidizing or reducing agent is necessary.
For example, primary alcohols may be oxidized to aldehydes in the presence of the oxidizing agent PCC (pyridinium chlorochromate):
Ketones undergo reduction to secondary alcohols in the presence of NaBH4 (sodium borohydride):
Bonds and Oxidation Levels
(arrange the common functional groups in order from least to most oxidized)
The more bonds a carbon has to an electronegative element, the more oxidized it is. Note: Halogens (X) count too as EN.
- The more bonds a carbon has to hydrogen, the more reduced it is.
- Hydrocarbons are in the lowest oxidation level.
Nucleophiles v. Electrophiles, (vs Inert)
- Nucleophiles are literally "things that love nuclei". Nuclei are positively-charged. Since positive charges attract them, nucleophiles are either negatively-charged, or at least have a negatively-polarized part that is reactive.
- Electrophiles "love electrons". They are either positively-charged or have a positively-polarized part.
Nucleophiles react with Electrophiles!
FG Reactivity Special Behavior #1:
Protonation of an Alcohol
Alcohols can react as either nucleophiles or electrophiles.
When an alcohol is protonated, it becomes positively-charged. It will then have a good leaving group (water) and may participate in a reaction as anelectrophile.
In the first step, the alcohol acts as a nucleophile. Because of the polarized C-O bond, the oxygen has a partial negative charge and will acquire a proton from HBr.
Once protonated, the molecule bears a good leaving group in H2O. The result is formation of a carbocation, which is electron-deficient. This carbocation will react electrophilically with Br-.
FG Reactivity Special Behavior #2:
Carbonyls as Electrophiles
Carbonyls can also react as either nucleophiles or electrophiles.
In the presence of a strong acid, the carbonyl compound will become protonated thus becoming an electrophile.
In this example, the carbonyl double bond is polarized. That is, the effect of the electronegative oxygen is to leave the carbonyl carbon with a small positive charge. As a result, the carbonyl carbon is electron-deficient and is susceptible to attack by an electron-rich species (a nucleophile).
Once the carbonyl acquires a proton, it can react with an appropriate nucleophile to form the corresponding addition product.
By now you will have reviewed the basic structure and properties of functional groups that will appear on Test Day.
Let's continue our review with a discussion of nomenclature.
IUPAC system, the names of organic molecules
- Locants are the numbers that tell where principal functional group and substituents are located on the main chain or ring.
- Substituent Prefixes identify what substituents are located on the main chain or ring.
- The Parent is the part that identifies the size of the main chain or ring.
- The Suffix identifies the compound's principal functional group.
Identify the locant, substituent prefix, parent and suffix in 5-methyl-2-heptanol.
In 5-methyl-2-heptanol, the locant 5 indicates the methyl group is a substituent of the fifth carbon in the parent hydrocarbon chain, which is 7 carbons long (heptane).
The principal functional group in this molecule is an alcohol, as indicated by the 'ol' suffix.
Note: the longest carbon chain must be numbered such that the principal functional group receives the lowest possible number.
IUPAC Suffixes for the Common Functional Groups
IUPAC Suffixes and Prefixes for the Common Functional Groups
How do you decide which suffix to use when there are two functional groups in a structure?
The more oxidized the functional group, the higher priority in the name.
- The suffix of the highest priority functional group is used as the ending, and this functional group gets the lowest possible number in the C-skeleton.
- Any other functional groups become substituents.
- On Test Day, you won't have to worry too much about cases where both functional groups have the same oxidation level.
In naming compounds, functional groups are assigned priorities according to the following order:
What is the name of the following compound?
The ketone functional group is more oxidized than the bromo- group. That is, the ketone group is the principal functional group in this molecule because it contains a C=O double bond.
The parent chain is pentane (5 carbon chain). The functional group with the highest priority is assigned the lowest possible number on the carbon chain. The parent chain contains bromo- and phenyl- substituents, both on C4.
common names of common molecules
What is the structure of acetic acid?
IUPAC: ethanoic acid
Formula: CH3COOH, (also written as CH3CO2H or C2H4O2)
For each common name, what is the corresponding structure?
Steps for formation of the systematic name for an organic compound
Identify the longest carbon chain containing the principal functional group.
- Identify all substituents.
- Number the carbon chain so that the principal functional group receives the lowest number. For alkanes, number the carbon atoms so that the substituents receive the lowest possible numbers.
- The IUPAC name is obtained by arranging the substituent prefixes in alphabetical order and attaching them to the name of the longest carbon chain.
IUPAC Nomenclature Rule #1:
Identify the longest carbon chain containing the principal functional group.
- When naming compounds, the longest continuous carbon chain within the compound is taken as the backbone.
- If there are two or more chains of equal length, the most highly substituted chain takes precedence.
- Number the chain from one end in such a way that the lowest set of numbers is obtained for the substituents.
Example (not always obvious):
Straight chain alkanes have the general formula CnH2n +2 (n is an integer)
Q: How many isomers exist with the formula C5H12?
- The first thing to recognize is that the formula C5H12 corresponds to a straight chain alkane (C5H2(5)+2).
- Start by drawing the simplest molecule, the one with the longest carbon chain and no branches.
- Now be systematic. Decrease the length of the carbon chain to 4 and determine the number of possible isomers.
- Now decrease the longest chain by one more carbon.
There are 3 isomers of C5H12.
Longer chain: consist of prefixes derived from the Greek root for the number of carbon atoms, with the ending -ane.
IUPAC Nomenclature Rule #2:
Identify all substituents.
Substituents are named according to their appropriate prefix with the ending -yl. More complex substituents are named as derivatives of the longest chain in the group.
- The prefix n- in the above example indicates an unbranched ("normal") compound.
- If there are two or more equivalent groups, the prefixes di-, tri-, tetra-, etc. are used.
- Each substituent is assigned a number to identify its point of attachment to the principal chain. If the prefixes di-, tri-, tetra-, etc. are used, a number is still necessary for each individual group.
Name the following molecule:
The compound is a butane because the longest carbon chain is 4 carbons long. The substituents are placed in alphabetical order and are assigned a number corresponding to their position on the carbon chain. We use 'di' to indicate there are 2 methyl group substituents.
Identify the substituents in the following molecule:
IUPAC Nomenclature Rule #4:
The IUPAC name is obtained by arranging the substituent prefixes in alphabetical order and attaching them together.
- List the substituents in alphabetical order with their corresponding numbers. Prefixes such as di-, tri-, etc., as well as the hyphenated prefixes (tert-sec-, n-) are ignored in alphabetizing.
- By contrast, cyclo-, iso-, and neo- are considered part of the group name and are alphabetized.
- Commas should be placed between numbers, and hyphens should be placed between numbers and words.
- Example: The longest carbon chain containing the principal functional group is 9 carbons long (nonane). The molecule contains methyl, ethyl, and tert-butyl substituents. The chain is numbered so the substituents receive the lowest possible numbers. Finally, we arrange the substituent prefixes in alphabetical order and attach them to the name of the longest carbon chain.
Alkanes can form rings.
These are named according to the number of carbon atoms in the ring with the prefix cyclo-.
Substituted cycloalkanes are named as derivatives of the parent cycloalkane. The substituents are named, and the carbon atoms are numbered around the ring starting from the point of greatest substitution.
The compound on the left is a cyclobutane because it is a ring containing 4 saturated carbons. Its substituents are methyl and ethyl groups. We number the ring so as to give the principal functional group the lowest number, so the carbon bearing the substituents is C1. Putting the substituents in alphabetical order gives 1-ethyl-1-methylcyclobutane.
The compound on the right is a cycloheptane because it's a ring made up of 7 saturated carbons. The substituents are an isopropyl group and two methyl groups. Numbering the ring so the substituents receive the lowest numbers gives: 4-isopropyl-1,1-dimethylcycloheptane.
- Alkenes are compounds containing carbon-carbon double bonds.
- Noncyclic alkenes have the general formula CnH2n (n is an integer).
- The nomenclature rules are essentially the same as for alkanes, except that the ending -ene is used rather than -ane.
- When identifying the carbon backbone, select the longest chain that contains the double bond (or the greatest number of double bonds, if more than one is present).
The backbone is numbered so that the double bond gets the lowest possible number.
Substituents are named as they are for alkanes, and their positions are specified by the number of the backbone carbon to which they are attached.
Alkenes take priority over alkanes, so our longest carbon chain must contain the alkene, which is the principal functional group in this molecule. In this case, the only substituent is an ethyl group. Next, number the carbon chain so the principal functional group gets the lowest number.
Finally, put it together. The 2 in front of ethyl indicates the ethyl group substitutes at the 2 position, while the 1 in front of pentene indicates the double bond is located between C1 and C2.
Notice we had to replace the "ane" ending with "ene" to indicate the alkene principal functional group.