Organic Chemistry - Nomenclature Notes (Alcohols, Diols, Cycloalkanes, Bicyclic Molecules, Alkenes)
Exam Preparation Reminder
A quiz will be held on Monday, covering all material up to and including today's lecture, which aims to complete nomenclature.
Alcohol Nomenclature
Definition: Compounds containing a hydroxyl (- ext{OH}) group.
General Naming Rules (IUPAC):
Identify the longest continuous carbon chain (parent chain) that contains the hydroxyl group.
Take the parent alkane name, drop the final "e", and add the suffix "-ol".
Specify the position of the hydroxyl group by placing a number (locant) before the parent name. This number should be the lowest possible for the hydroxyl group.
Example: For a four-carbon chain with the (- ext{OH}) on the second carbon, it's 2 ext{-butanol}. Note that the numbering gives the (- ext{OH}) group the lowest possible number.
Alternate Naming Convention:
The locant for the hydroxyl group can be placed directly before the "-ol" suffix (e.g., ext{butan}-2 ext{-ol}).
Priority Rules:
The hydroxyl group takes priority over alkyl groups and halogen (chloro, bromo, etc.) groups in assigning the lowest possible number to the carbon chain.
Numbering the chain should always aim to give the carbon bearing the hydroxyl group the lowest number.
Examples with Substituents:
ext{4-methylpentan-1-ol}. The parent chain is a five-carbon pentane. The (- ext{OH}) is at position 1, and a methyl group is at position 4. Numbering starts from the end closest to the (- ext{OH}) group.
ext{3-chloropropan-1-ol}. The parent is a three-carbon propane. The (- ext{OH}) is at position 1, and a chloro group is at position 3. The hydroxyl group dictates the numbering priority over the chloro group.
Common Naming Conventions (Alkyl Alcohol):
Alcohols can also be named by first naming the alkyl group attached to the hydroxyl group, followed by the word "alcohol".
Examples:
ext{Propyl alcohol} (for propan-1-ol).
ext{Butyl alcohol} (for butan-1-ol).
ext{Sec-butyl alcohol} (for butan-2-ol, where the (- ext{OH}) is on a secondary carbon).
ext{Tert-butyl alcohol} (for 2-methylpropan-2-ol, where the (- ext{OH}) is on a tertiary carbon).
ext{Isobutyl alcohol} (for 2-methylpropan-1-ol, with an 'iso' branched structure).
ext{Neopentyl alcohol} (for 2,2-dimethylpropan-1-ol, a five-carbon highly branched R group).
Diols (Alcohols with Multiple Hydroxyl Groups)
Naming: Compounds with more than one hydroxyl group are named as "alkane diols" (for two -OH groups), "triols" (for three -OH groups), etc. The "e" from the parent alkane name is retained, and the suffix "-diol" (or "-triol") is added.
Locants: The positions of all hydroxyl groups must be specified. These are included before the "-diol" suffix.
Alternate Naming: The locants can be placed right before the "-diol" ending (e.g., ext{propane-1,2-diol}.
Examples:
Ethane-1,2-diol: A two-carbon chain with (- ext{OH}) groups at positions 1 and 2. This is commonly known as ethylene glycol, used as antifreeze.
Propane-1,2-diol: A three-carbon chain with (- ext{OH}) groups at positions 1 and 2. Commonly known as propylene glycol.
Propane-1,3-diol: A three-carbon chain with (- ext{OH}) groups at positions 1 and 3. Known as trimethylene glycol. This name highlights the three methylene ( ext{CH}_2) units between the hydroxyls.
Cycloalkane Nomenclature
Basic Naming: For cyclic alkanes without substituents, simply add the prefix "cyclo-" to the corresponding alkane name.
Example: A three-carbon ring is ext{cyclopropane}. A six-carbon ring is ext{cyclohexane}.
With One Substituent:
Name the substituent, then the cycloalkane. No locant is needed, as the substituent is assumed to be at position 1.
Priority: If the ring has more carbons than the attached alkyl chain, the compound is named as a cycloalkane with the alkyl group as a substituent.
Example: An isopropyl group attached to a cyclohexane ring is ext{isopropylcyclohexane}.
Example: A chloro group attached to a cyclopentane ring is ext{chlorocyclopentane}.
With Two or More Substituents:
Two Substituents:
Assign numbers to the carbons starting with the carbon bearing the substituent that comes first alphabetically as position 1.
Number the ring in the direction (clockwise or counter-clockwise) that gives the other substituent the lowest possible number.
Example: For a cyclohexane ring with an ethyl group and a methyl group: the ethyl group comes first alphabetically, so it's assigned position 1. Numbering to give the methyl group the lowest number results in ext{1-ethyl-3-methylcyclohexane}.
More Than Two Substituents:
Number the ring such that all substituents are given the lowest set of numbers possible. Alphabetical order is used for naming after numbers are assigned.
Example: For a cyclohexane with chloro, ethyl, and methyl groups: The numbering is chosen to achieve the lowest possible sum of locants, even if it means an alphabetically lower substituent doesn't start at 1. If numbering yields 1,2,4 versus 1,3,4, choose 1,2,4. The full name is then ordered alphabetically: ext{4-chloro-1-ethyl-2-methylcyclohexane}.
Cyclic Groups as Substituents:
If the alkyl chain attached to the ring has more carbons than the ring itself, the compound is named as an alkane with the cyclic group as a substituent (a cycloalkyl group).
Example: A four-carbon cyclobutyl group attached to a five-carbon alkane chain would be ext{1-cyclobutylpentane} (the pentane chain is the parent).
Bicyclic Molecules Nomenclature
Definition: Molecules containing two fused rings, sharing at least two carbons.
Bridgehead Carbons: The carbons common to both rings, from which bridges originate.
Bridges: The carbon chains connecting the bridgehead carbons.
Naming Convention (Bicyclo[x.y.z]alkane):
Count the total number of carbons in the entire bicyclic system to determine the parent alkane name.
Prefix with "bicyclo-".
Inside square brackets, list the number of carbons in each of the three bridges (the paths connecting the bridgehead carbons), in decreasing order (x \ge y \ge z), separated by dots.
A "zero-carbon bridge" exists if the bridgehead carbons are directly connected without any intervening carbons.
Example: A bicyclic system with a total of seven carbons, having bridges of 2, 2, and 1 carbons, is named ext{bicyclo[2.2.1]heptane}.
Naming with Substituents:
Assign numbers starting from one bridgehead carbon.
Number the carbons along the longest bridge first, then the next longest, and finally the shortest bridge. This ensures the longest continuous numbering path.
Add the substituent's name and its assigned position to the bicyclic parent name.
Example: For a bicyclic octane with bridges of 3, 2, and 1 carbons, and a methyl group at position 8: ext{8-methylbicyclo[3.2.1]octane}. First, the largest bridge (3 carbons) is numbered, then the 2-carbon bridge, then the 1-carbon bridge.
Alkene Nomenclature
Basic Naming:
Identify the longest carbon chain containing the double bond.
Take the parent alkane name, drop the "-ane" suffix, and add "-ene".
Number the chain to give the double bond the lowest possible number. This number indicates the starting position of the double bond.
Example: A four-carbon chain with a double bond at position 1 is ext{but-1-ene} (or ext{1-butene}).
Example: A six-carbon chain with a double bond at position 2 is ext{hex-2-ene} (or ext{2-hexene}).
Priority Rules:
The double bond takes priority over alkyl groups and halogen groups in determining the numbering of the parent chain.
Example with Alkyl Substituents: For ext{2,5-dimethylhex-2-ene}, the longest chain containing the double bond is 6 carbons. The double bond is at position 2. Methyl groups are at positions 2 and 5. The double bond dictates numbering priority.
Example with Halogen Substituents: For ext{3-chloroprop-1-ene} (or ext{3-chloropropene}), the double bond at position 1 takes priority over the chloro group at position 3. For a three-carbon propene, the double bond position is often understood to be at 1 and can be omitted.
Tie-breaking Rule:
If the double bond can be assigned the same lowest number from either end of the chain, then number the chain to give any substituents the lowest possible numbers.
Example: In a four-carbon butene where the double bond is between carbons 2 and 3 (making it a ext{but-2-ene}), either numbering direction gives the double bond a locant of 2. If there's a chloro group, number from the end that gives the chloro group the lowest number (e.g., ext{1-chlorobut-2-ene}).
Common Alkene Names:
Ethene: Usually called ethylene ( ext{CH}2= ext{CH}2).
Propene: Usually called propylene ( ext{CH}3 ext{CH}= ext{CH}2).
2-Methylpropene: Often called isobutylene (a branched alkene with a double bond, analogous to isobutane structure).
Upcoming Material
The lecture concludes here but is stated to be responsible upto this point for the quiz on Monday. The next part of the nomenclature will be less tedious. The content covered includes alcohols, diols, cycloalkanes, bicyclic molecules, and alkenes.