Alicyclic Compounds & Synthesi

Cheatsheet Content

### Alicyclic Compounds - **Definition:** Cyclic hydrocarbons whose properties are similar to open-chain aliphatic hydrocarbons (alkanes, alkenes, alkynes). They contain one or more rings but do not show aromatic character. - **Example:** Cyclobutane ``` H₂C-CH₂ | | H₂C-CH₂ ``` ### Nomenclature of Alicyclic Compounds - Use the "Cyclo" prefix. - Triangle: Cyclopropane - Square: Cyclobutane - Hexagon: Cyclohexane - **Substituents:** If a substituent is present, the carbon atom bearing it gets the lowest possible number. - Example: Chlorocyclopropane, 1,2-dichlorocyclopentane, 1-ethyl-2-methylcyclohexane - **Multiple Bonds:** If multiple bonds are present, they get the lowest number. - Example: 4-ethylcyclopentene, 1-methylcyclopentene, cyclohexa-1,3-diene - **Functional Groups:** Priority order: Functional group > Multiple bond > Substituent. - Example: cyclohex-3-enol, (5,6-dimethylcyclohex-2-en-1-ol), 1-methylcyclopentanol, 3-hydroxycyclohexanone ### Bicyclic Compounds - **Definition:** Compounds containing two fused rings that share two or more carbon atoms. Named as bicycloalkanes. They are a specific type of polycyclic compounds. - **Bridgehead Carbons:** The common atoms connecting the rings in a bicyclic compound. - **Classification:** - **Fused:** Bridgehead carbons are adjacent. - **Bridged:** Bridgehead carbons are not adjacent. - **Spirocyclic Compounds:** The extreme case where there is only one common carbon atom shared between two rings. ### Naming Bicyclic Compounds - **Numbering Rule 1:** Start numbering from one of the bridgehead carbon atoms. - **Numbering Rule 2:** Number through the largest ring first, then the second largest, and so on. - **Parent Chain:** Based on the total number of carbon atoms in all rings (e.g., nonane for 9 carbons). - **Format:** `Bicyclo[#C, #C, #C]Parent Chain` - Numbers in brackets indicate the number of carbon atoms in each bridge (ring section) *excluding* the bridgehead carbons, arranged in decreasing order. - A "zero" in the brackets specifies there are only two rings (e.g., bicyclo[4.3.0]nonane). - If there's a third bridge of 1 carbon, it's included (e.g., bicyclo[3.3.1]nonane). ### Bicyclic Compounds with Substituents - The presence of substituents does not change the main numbering rule: start from a bridgehead and move through the largest ring first. - Substituents are placed at the beginning of the name, like in alkanes. - **Example:** 6-bromobicyclo[3.2.1]octane (largest ring numbered first, even if it doesn't give the substituent the lowest number). ### Ring Priority in Bicyclic Compounds - **Step 1:** Find the largest ring (ring with the most carbon atoms). This gets priority 1. - **Step 2:** If rings have the same number of carbon atoms, the one with a substituent gets priority. ### Preparation of Alicyclic Compounds #### From Dihalogen Compounds - Dihalogen compounds react with sodium (Na) or zinc (Zn) metal to form corresponding cycloalkanes. - **Example (Zn):** $$ \text{H}_2\text{C} \\ \quad\vert \\ \text{H}_2\text{C}-\text{Cl} \\ \quad\vert \\ \text{H}_2\text{C}-\text{Cl} \\ \quad\quad\text{(1,3-dichloropropane)} $$ $$ \xrightarrow{\text{+ Zn, }\Delta} $$ $$ \text{CH}_2 \\ \quad\diagup\diagdown \\ \text{CH}_2-\text{CH}_2 \\ \quad\quad\text{(cyclopropane)} + \text{ZnCl}_2 $$ - **Example (Na):** $$ \text{H}_2\text{C}-\text{Br} \\ \quad\vert \\ \text{H}_2\text{C}-\text{C}-\text{Br} \\ \quad\vert \\ \text{H}_2\text{C}-\text{Br} $$ $$ \xrightarrow{\text{+ 2Na, }\Delta} $$ $$ \text{H}_2\text{C}-\text{CH}_2 \\ \quad\vert \\ \text{H}_2\text{C}-\text{CH}_2 \\ \quad\quad\text{(cyclobutane)} + \text{2NaBr} $$ #### From Diels-Alder Reaction - Two unsaturated molecules (a diene and a dienophile) combine to form a cyclic compound. - **Example:** $$ \text{Diene} + \text{Dienophile} \rightarrow \text{Cyclohexene} $$ $$ \text{1,3-butadiene} + \text{ethene} \rightarrow \text{cyclohexene} $$ $$ \text{Cyclopentadiene} + \text{ethene} \rightarrow \text{Bicyclo[2.2.1]hept-2-ene} $$ #### By Reduction of Aromatic Compounds - Hydrogenation (reduction with H$_2$/Ni) of aromatic compounds yields alicyclic compounds. - **Example:** $$ \text{Benzene} \xrightarrow{\text{H}_2\text{ / Ni, }\Delta} \text{Cyclohexane} $$ $$ \text{Phenol} \xrightarrow{\text{H}_2\text{ / Ni, }\Delta} \text{Cyclohexanol} $$ ### Baeyer's Strain Theory - **Proposed by:** Adolf Baeyer in 1885 to explain the relative stability of cycloalkanes. - **Assumptions:** 1. All cyclic rings are planar (carbon atoms lie in the same plane). 2. The ideal bond angle for a tetrahedral carbon atom is $109^\circ 28'$. 3. Any deviation from this ideal angle develops strain (angle strain) on the ring, decreasing stability. - **Angle Strain (d):** Calculated as $d = \frac{1}{2}(109^\circ 28' - \text{bond angle of ring})$. ### Angle Strain Calculations & Stability - **Cyclopropane:** (Equilateral triangle, bond angle $60^\circ$) $$ d = \frac{1}{2}(109^\circ 28' - 60^\circ) = 24^\circ 44' $$ - Highest angle strain, predicted to be most unstable. (Matches experimental observation: undergoes ring opening). - **Cyclobutane:** (Square, bond angle $90^\circ$) $$ d = \frac{1}{2}(109^\circ 28' - 90^\circ) = 9^\circ 44' $$ - Less strain than cyclopropane, predicted to be more stable. (Matches experimental observation). - **Cyclopentane:** (Bond angle $108^\circ$) $$ d = \frac{1}{2}(109^\circ 28' - 108^\circ) = 0^\circ 44' $$ - Minimum angle strain, predicted to be most stable. (Matches experimental observation: low reactivity). - **Cyclohexane:** (Bond angle $120^\circ$) $$ d = \frac{1}{2}(109^\circ 28' - 120^\circ) = -5^\circ 16' $$ - Negative strain indicates "expansion" from ideal angle. Predicted to be less stable than cyclopentane. - **Cycloheptane:** (Bond angle approx $128.5^\circ$) $$ d = \frac{1}{2}(109^\circ 28' - 128.5^\circ) \approx -9^\circ 33' $$ - Increasing negative strain. | Cycloalkane | Angle Strain (d) | | :------------ | :--------------- | | Cyclopropane | $+24^\circ 44'$ | | Cyclobutane | $+9^\circ 44'$ | | Cyclopentane | $+0^\circ 44'$ | | Cyclohexane | $-5^\circ 16'$ | | Cycloheptane | $-9^\circ 33'$ | ### Limitations of Baeyer's Theory - **Incorrect Assumption:** Baeyer's theory assumes planar rings. In reality, rings larger than cyclopropane (especially cyclohexane) adopt puckered conformations (e.g., chair, boat for cyclohexane) to relieve angle strain and torsional strain. - **Discrepancy with Experiment:** - Theory predicts cyclohexane and higher cycloalkanes should be increasingly unstable due to increasing angle deviation. - **Experimental Observation:** Cyclohexane and higher cycloalkanes are found to be quite stable and do not undergo ring-opening reactions, contradicting the theory. - **Applicability:** The theory satisfactorily explains the stability of lower cycloalkanes (cyclopropane, cyclobutane) but fails for cyclohexane and higher cycloalkanes.