SA

Organic Chemistry Naming and Reactions Review

  • Naming Cycloalkanes, Alkenes, and Alkynes

    • General Rules for Rings

      • Count the number of points (carbons) in the ring: 5 carbons = pent, 6 carbons = hex, 8 carbons = oct.

      • Add the prefix "cyclo" before the parent name unless the ring has a specific name.

      • When numbering carbons in a ring, every step must be considered.

    • Substituent Naming and Numbering

      • If there's only one substituent, that carbon is carbon number one.

      • If there are multiple substituents, they are placed in alphabetical order.

      • The numbering should result in the lowest possible carbon numbers for all substituents.

      • Tie-breaking Rule: If multiple numbering schemes result in the same lowest initial numbers, choose the scheme that gives the lowest sum of all substituent carbon numbers.

      • Alphabetical Order Priority: When deciding which carbon gets number one, alphabetical order of substituents typically determines the priority if the lowest sum rule doesn't differentiate.

      • Prefixes like "di-" or "tri-" (e.g., in dimethyl) do not count towards alphabetical ordering.

    • Double and Triple Bonds in Rings (Cycloalkenes & Cycloalkynes)

      • The carbons involved in the double or triple bond must always be assigned the lowest possible numbers, specifically C1 and C2.

      • C1 and C2 must share the double or triple bond.

      • The numbering direction (clockwise or counter-clockwise) is chosen to give substituents the lowest possible numbers after assigning C1 and C2 to the bond.

    • Naming Halogens as Substituents

      • When halogens (Group 17) act as substituents, their names change:

        • Chlorine (Cl) becomes chloro.

        • Fluorine (F) becomes fluoro.

        • Bromine (Br) becomes bromo.

        • Iodine (I) becomes iodo.

      • Fluorine is the most reactive element on the periodic table.

    • Examples

      • 1-ethyl-1-methylcyclopentane: Five carbons in the ring (cyclopentane), one ethyl group, one methyl group. Ethyl comes before methyl alphabetically, so if both were on the same carbon, that carbon would be C_1. If on different carbons like in the example discussed, the one that results in the lowest sum for substituents would be prioritized.

      • Cyclohexene: A six-carbon ring with a double bond. The double bond is between C1 and C2. It can be named as 1-cyclohexene though often the '1' is omitted if there are no other substituents because the double bond is assumed to be at C_1.

      • 3,3-dimethylcyclopentene: A five-carbon ring with a double bond. If the double bond is between C1-\$C2, and there are two methyl groups at C3, the name is 3,3-dimethylcyclopentene. The numbering must give the double bond the lowest numbers (C1, C2) and subsequently the substituents the lowest numbers (C3,C3 vs. potentially C5 if counted the other way).

      • 4,4-dichloro-1-methylcyclopentene: Five-carbon ring, double bond is C1-\$C2. A methyl group at C1 and two chloro groups at C4. The numbering prioritizes the lowest sum for substituents (1+4+4 = 9 if methyl is C1, vs. 2+4+4 = 10 if methyl is C2 through other counting), and chloro comes before methyl alphabetically.

      • Cyclooctyne: Eight-carbon ring with a triple bond. Numbering starts at one of the carbons in the triple bond. If no substituents, the '1' is often omitted. e.g. cyclooctyne (implies 1-cyclooctyne).

      • 3,3-difluorocyclooctyne: Eight-carbon ring with a triple bond (C1, C2). Two fluoro groups at C_3. The name is 3,3-difluorocyclooctyne.

      • 3-ethylcyclopropine / 3-ethylcycloprop-1-yne: A three-carbon ring with a triple bond. An ethyl group at C_3. IUPAC allows both forms, sometimes leading to convention debates.

      • 3-methylcyclobut-1-yne: A four-carbon ring with a triple bond between C1-\$C2. A methyl group at C_3. The name can be 3-methylcyclobutyne or more explicitly 3-methylcyclobut-1-yne.

  • Cis/Trans Isomers (Geometric Isomers)

    • Definition: Isomers that have the same chemical formula but different spatial arrangements of atoms due to restricted rotation around a double bond. This applies to alkenes and cycloalkenes.

    • Representations

      • Cis isomer: Substituents (identical elements or groups) are on the same side of the double bond (e.g., both above or both below).

      • Trans isomer: Substituents (identical elements or groups) are on opposite sides of the double bond (one above, one below).

    • Conditions for Cis/Trans Isomerism: There must be different groups attached to each carbon of the double bond. The comparison is made between similar groups on either side of the double bond. They can be hydrogens or other identical elements/groups.

    • Importance: Cis/trans isomerism can have significant clinical implications, where one form (e.g., cis) may be an effective medication, while the other (e.g., trans) could be a placebo or even harmful.

    • Examples

      • Cis-1,2-dichloroethene: Two carbons with a double bond. Chlorines are on the same side.

      • Trans-2,3-dibromobutene: A four-carbon chain (butene) with a double bond between C2 and C3. Bromines are attached to C2 and C3 and are on opposite sides of the double bond. The name must specify the positions of bromines: 2,3-dibromo.

      • Cis-2,3-dichlorobutene: A four-carbon chain (butene) with a double bond between C2 and C3. Chlorines are attached to C2 and C3 and are on the same side of the double bond.

      • Trans-2-butene: A four-carbon chain with a double bond between C2 and C3. If no other substituents are mentioned, the comparison is made with hydrogens, which are on opposite sides of the double bond.

  • Addition Reactions

    • Definition: Reactions where atoms are added across a double or triple bond, converting it to a single bond (or a double bond in the case of alkynes reacting with one equivalent of reagent).

    • Types of Addition Reactions

      • 1. Hydrogenation (Adding Hydrogen)

        • Purpose: Converts unsaturated fats (oils, double bonds) into saturated fats (solids, single bonds) to increase spreadability and firmness at room temperature (e.g., in butter substitutes).

        • Reactants: An alkene or alkyne and hydrogen gas (H_2).

          • For alkenes: Alkene + H_2

          • For alkynes: Alkyne + 2H_2 (two molecules of diatomic hydrogen).

        • Catalyst: Requires a metal catalyst, typically Platinum (Pt), Palladium (Pd), or Nickel (Ni). Only one catalyst is needed.

        • Product: Converts alkenes/alkynes into alkanes (all single bonds). The number of carbons in the parent chain does not change.

        • Mechanism: The double/triple bond breaks, and hydrogen atoms add to the carbons that were part of the multiple bond, satisfying carbon's four-bond requirement.

        • Examples

          • Ethene + H2 \xrightarrow{Pt} Ethane (CH2=CH2 to CH3-CH_3)

          • 1-Butene + H_2 \xrightarrow{Pt} Butane

          • Cyclobutene + H_2 \xrightarrow{Pd} Cyclobutane

          • 3-Hexene + H_2 \xrightarrow{Ni} Hexane

          • Cyclooctyne + 2H_2 \xrightarrow{Pt} Cyclooctane

      • 2. Hydration (Adding Water)

        • Purpose: Converts alkenes into alcohols.

        • Reactants: An alkene and water (H_2O or HOH).

        • Catalyst: Requires a strong acid catalyst, indicated by H^+ or specific strong acid formulas like HI or H2SO4. A single arrow in the reaction indicates a strong acid.

        • Product: An alcohol, distinguished by a hydroxide (OH) substituent attached to one of the carbons.

        • Naming Alcohols: Drop the '-e' from the corresponding alkane name and add '-ol' (e.g., Ethane becomes Ethanol, Propane becomes Propanol). The carbon number of the hydroxide group is specified (e.g., 2-Propanol).

        • Markovnikov's Rule (for Hydration): When water adds across an unsymmetrical double bond:

          • The hydrogen atom (H) from water adds to the carbon atom of the double bond that already has more hydrogen atoms attached to it.

          • The hydroxide group (OH) from water adds to the carbon atom of the double bond that has fewer hydrogen atoms attached to it.

        • Examples

          • Ethene + H_2O \xrightarrow{H^+} Ethanol

          • 1-Butene + H2O \xrightarrow{H^+} 2-Butanol (Hydroxide adds to C2 as it has fewer hydrogens than C_1)

          • Propene + H2O \xrightarrow{H^+} 2-Propanol (Hydroxide adds to C2)

          • Hexene + H2O \xrightarrow{H2SO4} 2-Hexanol (Hydroxide adds to C2$$ on the long chain example, applying Markovnikov's rule)