Organic Chemistry Nomenclature Review - Ch 1-7

Naming Alkyl Groups

Methyl Group: A substituent consisting of one carbon atom. (e.g., meth becomes methyl when a substituent).
Alphabetical Order for Substituents: When listing multiple different substituents in a chemical name, they are ordered alphabetically. Prefixes like di-, tri-, tetra- are not considered for alphabetical ordering; the primary name of the substituent is used (e.g., methyl for dimethyl, ethyl for ethyl).
- Example: In 4-ethyl-2-methylhexane, ethyl comes before methyl because 'e' precedes 'm', despite the locants.

Prefixes for Multiple Identical Substituents

Purpose: To simplify naming when multiple identical groups are present, prefixes are used instead of listing each group separately.
Prefixes:
- \text{di-}: Two identical groups (e.g., dimethyl)
- \text{tri-}: Three identical groups (e.g., trimethyl)
- \text{tetra-}: Four identical groups (e.g., tetramethyl)
Locant Rule for Multiple Identical Groups: When multiple identical groups are attached, all of their locants must be specified, even if they are on the same carbon.
- Example: If two methyl groups are on carbon 2, it's named as 2,2-dimethyl.
Alphabetical Ordering Exception: As noted above, these prefixes (di-, tri-, tetra-) are ignored when determining alphabetical order for the overall name. The alphabetical order is based on the root name of the substituent (e.g., 'm' for methyl in dimethyl).

Determining the Parent Chain and Numbering

Identify the Longest Continuous Carbon Chain: This chain forms the parent name of the compound (e.g., butane, hexane, heptane).
Identify and Circle Substituents: Clearly mark all groups attached to the parent chain to ensure they are included in the name.
Numbering Rule 1: Lowest Locant for First Substituent: Number the parent chain from the end that gives the lowest possible number to the first substituent encountered.
- Example: For 2,3-dimethylbutane, numbering from left-to-right (1-2-3-4) gives substituents at positions 2 and 3. Numbering from right-to-left (4-3-2-1) also gives substituents at positions 2 and 3. In this symmetrical case, the name remains the same.
Numbering Rule 2 (Tie-breaker for First Substituent): If numbering from both ends yields the same locant for the first substituent, then you must examine the second (and subsequent) substituents to break the tie. Choose the numbering scheme that gives the lowest number to the next encountered substituent.
- Example: A hexane chain with methyl groups.
  - If numbering from the left gives locants 2, 4, 5.
  - If numbering from the right gives locants 2, 3, 5.
  - The right-to-left numbering is chosen because 3 < 4 for the second substituent.
  - Name: 2,3,5-trimethylhexane (not 2,4,5-trimethylhexane).

Selecting the Parent Chain in Complex Cases

Rule: If there are multiple continuous carbon chains of the same longest length, choose the chain that has the highest number of substituents attached to it.
- Example: A molecule where a straight 7-carbon chain has four substituents, but another 7-carbon path has only three substituents, and yet another has two. The straight chain with four substituents is preferred.
Final Naming Steps for Complex Alkanes:
1. Determine the longest chain with the most substituents.
2. Number the chain according to preference rules (lowest locant for first substituent).
3. Identify and name all substituents, applying prefixes for identical groups.
4. List substituents alphabetically (ignoring di-, tri-, etc.) and include their locants.
5. Combine into the full IUPAC name.
- Example: A heptane parent chain with methyl groups at positions 2, 3, 5 and a propyl group at position 4.
  - Methyl ('m') comes before propyl ('p') alphabetically (ignoring 'tri-').
  - Name: 2,3,5-trimethyl-4-propylheptane.

Common Alkyl Group Names

These common names are frequently encountered and often replace formal IUPAC naming for simple substituents.

Propyl Groups (3 Carbons):
- Propyl (or n-propyl): Formed by removing a primary hydrogen from a terminal carbon of propane. Point of attachment is primary.
- Isopropyl: Formed by removing a secondary hydrogen from the central carbon of propane. Point of attachment is secondary.
  - IUPAC Alternate for Isopropyl: 1-methylethyl (The carbon at the point of attachment is 1, the longest chain attached to it is ethyl, and there's a methyl group on carbon 1 of that chain: 1-methylethyl).
Butyl Groups (4 Carbons):
- n-Butyl: Formed by removing a primary hydrogen from a terminal carbon of a straight butane chain. Point of attachment is primary.
- Isobutyl: Derived from isobutane. Formed by removing a primary hydrogen from one of the terminal carbons of isobutane. Point of attachment is primary.
- Sec-butyl (s-butyl): Formed by removing a secondary hydrogen from a secondary carbon of butane. Point of attachment is secondary.
- Tert-butyl (t-butyl): Formed by removing a tertiary hydrogen from the central carbon of isobutane (a tertiary carbon). Point of attachment is tertiary.
Pentyl Groups (5 Carbons):
- Neopentyl: A specific branched 5-carbon structure. Neopentyl is a common name.
  - IUPAC Alternate for Neopentyl: 2,2-dimethylpropyl (Carbon at point of attachment is 1, longest chain from it is propyl, with two methyl groups at carbon 2 of that chain). Remember to use brackets or parentheses around complex substituent names in the full compound name.

Naming Complex Substituents (IUPAC Method)

Rule: When a substituent itself is branched and complex, it can be named using a miniature IUPAC system:
1. Assign number 1 to the carbon at the point of attachment to the main parent chain.
2. Find the longest carbon chain within that substituent starting from the point of attachment (carbon 1).
3. Name any further substituents on this internal chain, using their locants relative to carbon 1. For example, an ethyl chain with a methyl on the first carbon is 1-methylethyl.
4. Enclose the entire name of the complex substituent in parentheses or brackets when integrating it into the full compound name.
- Example (re-emphasizing Neopentyl): For the neopentyl group, the point of attachment is carbon 1. The longest chain from this carbon is a 3-carbon propyl chain. On the second carbon of this propyl chain, there are two methyl groups. Thus, its IUPAC name is 2,2-dimethylpropyl.

Alkyl Halide Nomenclature

There are two primary methods for naming alkyl halides:

1. Haloalkane Nomenclature (IUPAC Preferred for Complexity)

Method: The halogen is named as a prefix ending in '-o', followed by the alkane parent name.
Halogen Prefixes:
- Fluorine (\text{F}): Fluoro
- Chlorine (\text{Cl}): Chloro
- Bromine (\text{Br}): Bromo
- Iodine (\text{I}): Iodo
Rules:
1. Identify the longest carbon chain that contains the halogen.
2. Number the chain to give the lowest possible locant to the first substituent encountered, whether it's an alkyl group or a halogen.
3. Priority Rule (Halogen vs. Alkyl Tie-breaker): If numbering from both ends gives the same locant for the first encountered substituent, but one is a halogen and the other is an alkyl group, the halogen takes priority for the lowest locant.
  - Example: A pentane chain with a chloro group and a methyl group.
    - If numbering from left gives Chloro at 2 and Methyl at 4.
    - If numbering from right gives Methyl at 2 and Chloro at 4.
    - Since the halogen (chloro) can receive the lower locant (2), that numbering (left-to-right) is preferred.
    - Name: 2-chloro-4-methylpentane.

2. Alkyl Halide Nomenclature (Common Naming)

Method: The alkyl group is named first, followed by the halide name.
Halide Suffixes:
- Fluorine (\text{F}): Fluoride
- Chlorine (\text{Cl}): Chloride
- Bromine (\text{Br}): Bromide
- Iodine (\text{I}): Iodide
Examples:
- Ethyl chloride (for \text{CH}3\text{CH}2\text{Cl})
- Isopropyl bromide (for isopropyl group with \text{Br})
- t-Butyl bromide (for tert-butyl group with \text{Br})
- Isobutyl chloride (for isobutyl group with \text{Cl})
- Neopentyl bromide (for neopentyl group with \text{Br})