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Chapter 3. Alkanes

Recommended Article: 【Organic Chemistry】 Organic Chemistry Table of Contents

1. Overview

2. Nomenclature

3. Degree of Unsaturation

4. Physical Properties of Alkanes

5. Conformers

6. Cyclohexane

7. Alkane Reactions

1. Overview

⑴ Definition: Compounds composed only of carbon and hydrogen with single bonds

⑵ Hydrocarbon: Compounds composed only of carbon and hydrogen

① Type 1. Saturated hydrocarbons

② Type 1-1. Saturated hydrocarbons: Hydrocarbons without double or triple bonds. Includes alkanes

○ Paraffinic hydrocarbons: Can be represented as C_nH_2n+2. Examples: ethane, methane

○ Napthenic hydrocarbons: Ring-shaped hydrocarbons. Can be represented as CnH2n. Examples: cyclohexane

③ Type 1-2. Unsaturated hydrocarbons: Hydrocarbons with double or triple bonds. Includes alkenes, alkynes

○ Olefinic hydrocarbons: Hydrocarbons with one double bond. Can be represented as CnH2n

④ Type 2. Aromatic hydrocarbons: Hydrocarbons containing aromatic rings. Includes benzene

⑶ Major Alkanes

Table. 1. Major Alkane Properties

① C1-C2: Can be liquefied at very low temperatures. Transported as liquids in special cryogenic tanks. Used as liquefied natural gas (LNG)

② C3-C4: Easily liquefied at moderate pressure and room temperature. Used as liquefied petroleum gas (LPG)

③ C5-C8: Volatile liquids with good flow characteristics. Used as gasoline

④ C9-C16: Somewhat viscous liquids with high boiling points. Used as diesel fuel, heating oil, and jet fuel

⑤ C17 and above: Highly viscous liquids or waxy solids. Used as lubricating oil, heating oil, and more

2. Nomenclature

⑴ IUPAC Systematic Naming

① Prefix - Locant - Parent - Suffix

② Prefix: Indicates the type and number of substituents and their positions on the parent chain

③ Locant: Designates the position of the main functional group

④ Parent: Indicates the number of carbon atoms in the parent chain

⑤ Suffix: Indicates the functional group present in the compound, such as alkane, alkene, alkyne

⑵ Straight-Chain Alkane Nomenclature

① Selecting the main chain

○ Choose the longest continuous carbon chain as the main chain and treat the rest as substituents (side chains)

○ If there are multiple chains of the same length, choose the one with the most substituents as the main chain

② Substituent naming

○ Here, assume that only alkyl groups and halogen groups are present in the substituents.

○ Alkyl group: If alkane or cycloalkane acts as a substituent, replace -ane with -yl

○ Alkyl groups can be represented as R groups

○ Some R groups are represented with abbreviations (e.g., me for methyl, et for ethyl, pro for propyl)

○ alkyl (e.g., ethyl), alkenyl (e.g., ethenyl), and alkynyl (e.g., ethynyl) have equal priority

○ Common names: isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, neopentyl

○ Halogen groups: If a halogen element is a substituent, replace it with -o

○ Examples: fluoro, chloro, bromo, iodo

○ If the same substituent appears more than once, use prefixes to indicate their number

○ Examples: di(2), tri(3), tetra(4), penta(5), hexa(6)

③ Numbering

○ Assign sequential numbers to carbons along the main chain from one end to the other

○ When there is only one substituent, give the lowest number to the carbon bearing the substituent

○ When multiple substituents are present, assign numbering such that the lowest number is given to a certain substituent. Even if the sum of the numbers increases significantly, it remains a valid nomenclature.

○ Example: 2,6,6-trimethyloctane (O)

○ Example: 3,3,7-trimethyloctane (X)

○ If numbers are the same, prioritize alphabetical order for the substituents

○ Example: 3-ethyl-5-methylheptane

④ Complete Naming

○ List the substituents in alphabetical order

○ Examples: 4-ethyl-3-methyloctane

○ Prefixes like di(2), tri(3), etc., aren’t included in the alphabetical order

○ Example: 4-ethyl-2,3-dimethylheptane (O)

○ Example: 2,3-dimethyl-4-ethylheptane (X)

○ Exception: Exceptionally, if a substituent has another substituent, include it in the alphabetical order.

○ Example: 7-(2,2-dimethylbutyl)-4-ethyldodecane

○ Consider prefixes like iso, neo, cyclo for alphabetical order if using common names for substituents

○ Example: isopropyl > methyl

○ Ignore hyphenated prefixes like sec-, tert- when determining alphabetical order

○ Example: sec-butyl > ethyl

○ Named as 《(substituent position-substituent name)_n + main chain + suffix》.

○ However, for alkanes, the suffix is -ane.

⑤ For complex substituents, consider them as a single substituent. Enclose the substituent name in parentheses and prioritize alphabetical order

○ The carbon in the substituent attached to the main chain is labeled as carbon 1.

○ Examples: 4-(2-methylhexyl)octane

○ Examples: 7-(2,2-dimethylbutyl)-4-ethyldecane

⑶ Nomenclature of Cyclic Alkanes

① When only cyclic alkanes are present

○ Prefix “cyclo” is added to the name of the alkane that corresponds to the number of carbons in the ring

○ Examples: cyclopropane, cyclobutane, cyclopentane, cyclohexane, cycloheptane

② Naming the main chain

○ According to the previous IUPAC guidelines

○ Case 1: When both acyclic and cyclic alkane structures coexist: Represent the cyclic alkane as a substituent in the following cases:

○ When the substituent has a larger number of carbons

○ When multiple rings are present

○ When neither can be chosen as the parent structure

○ Case 2: When the number of carbons in the acyclic alkane chain and the cyclic alkane are the same, consider the cyclic alkane as the parent structure.

○ Latest IUPAC Guidelines: In cases where both ring and chain structures are present, the ring is chosen as the parent structure.

○ Example: octylcyclobutane (preferred)

③ When a cyclic alkane is the main chain

○ If there is only one substituent, numbering is not required

○ Example: methylcyclohexane

○ When multiple numbering methods are possible, assign numbers so that at least one is the smallest possible number (ref).

○ Example: 1-chloro-2,2,3-trimethylcyclopropane (X), 2-chloro-1,1,3-trimethylcyclopropane (O)

○ When there are two or more substituents, name the substituents, number the positions, and then name the entire molecule.

○ The carbon attached to the substituent that comes first in alphabetical order is numbered 1.

○ Example: 1-ethyl-2-methylcyclopentane

④ When an acyclic alkane is the main chain: Include the ring position number

○ Example: 1-cyclobutylpentane

⑷ Nomenclature of Complex Cyclic Alkanes

① Bicyclic alkanes or bridged rings

○ Investigate the path from one bridgehead carbon to another.

○ Count the number of carbons excluding the bridgehead carbon in each path and write it as [x.y.z] in decreasing order.

○ Numbering:

○ Step 1. Number each carbon starting from one at each bridgehead, following the longest path.

○ Step 2. Number the largest ring first.

Figure 1. Nomenclature of Bicyclic Alkanes

○ Step 3. In cases where multiple possibilities compete, assign numbers such that any carbon with a substituent gets a lower number.

Figure 2. Nomenclature of Bicyclic Alkanes

○ Naming format: 《(substituent position + substituent name)_n + bicyclo[x.y.z]alkane》, where alkane indicates the total number of carbons.

○ Example: 3,3-dimethylbicyclo[3.2.1]octane

② Fused rings (fused ring) (e.g., steroids)

○ Investigate the path from one bridgehead carbon to the other bridgehead carbon

○ Count the number of carbon atoms excluding the bridgehead carbon in each path and write it in descending order as [x.y.0].

○ Fused rings have directly connected shared carbons, always represented as z = 0.

○ Numbering: Proceed as in ①

○ Format: 《(substituent position + substituent name)_n + bicyclo[x.y.0]alkane》, where “alkane” indicates the total number of carbons

○ Example: 8-methylbicyclo[4.3.0]nonane

③ Spiro rings (spiro ring)

○ Investigate the path from one bridgehead carbon to itself

○ Count the number of carbon atoms excluding the bridgehead carbon in each path and write it in ascending order as [x.z].

○ Different from bicycloalkanes and fused rings due to not having a bridgehead carbon at 1.

○ Numbering: Proceed as in ①

○ Format: 《(substituent position + substituent name)_n + bicyclo[x.z]alkane》, where “alkane” indicates the total number of carbons

○ Example: 1-bromo-3-chlorospiro[4.5]decan-7-ol

⑸ Order

① Carbon Order: The number of carbons surrounding a specific carbon

② Hydrogen Order: The number of hydrogens bonded to a carbon

③ Butyl group

○ Butyl groups include n-butyl, isobutyl (2-methylpropyl), sec-butyl (1-methylpropyl), tert-butyl (1,1-dimethylethyl)

○ sec-butyl: When the attaching carbon of the sec-butyl group is a secondary carbon

○ tert-butyl: When the attaching carbon of the tert-butyl group is a tertiary carbon

3. Degree of Unsaturation

⑴ Degree of unsaturation or I.H.D. (index of hydrogen deficiency): Used to predict structures with multiple bonds or ring structures

⑵ Basic formula

⑶ Additional rules

① Halogen elements: Consider as hydrogen for analysis

② Oxygen: Disregard for analysis

③ Nitrogen: Subtract one hydrogen per nitrogen for analysis

⑷ Tip: The degree of unsaturation for a ring is equal to the minimum number of bonds that need to be broken until the ring is absent

① The reason the degree of unsaturation for adamantane is 3 can be understood using this tip

4. Physical Properties of Alkanes

⑴ Boiling Point

① Alkanes have lower boiling points than similarly sized polar molecules

② Increase in carbon number → Increase in surface area → Increase in intermolecular forces → Increase in boiling point

③ As surface area increases, boiling point increases: Alkanes without branches have higher boiling points among alkanes with similar molecular weights

○ Example: neopentane (CH₃)₄C (9.5 ℃) < isopentane (CH₃)₂CHCH₂CH₃ (27.8 ℃)

⑵ Melting Point (Freezing Point)

① Alkanes have lower melting points than similarly sized polar molecules

② Increase in carbon number → Increase in surface area → Increase in intermolecular forces → Increase in melting point

③ Symmetry: Increased symmetry leads to higher melting points for isomers

○ Example: neopentane (CH₃)₄C (-16.5 ℃) > 2,2-dimethylbutane (-98 ℃) > isopentane (CH₃)₂CHCH₂CH₃ (-160 ℃)

○ Reverse order compared to boiling point

④ Alkanes with an even number of carbons form more regular crystalline lattice structures than those with an odd number, resulting in higher melting points.

○ 2n → 2n + 1: The increase in melting point is small.

○ 2n + 1 → 2n + 2: The increase in melting point is large.

○ Even ethane has a higher melting point than propane.

⑶ Solubility

① Alkanes dissolve in organic solvents

② Alkanes are insoluble in water

5. Conformational Isomers (Conformers)

⑴ Overview

① Conformational isomers: Exhibiting various conformations due to free rotation of single bonds

② Conformational isomers are not strict isomers.

③ Cyclic alkanes, alkenes, and alkynes cannot freely rotate.

④ Newman projection: A model introduced for analyzing conformational isomers

○ The central dot represents the front carbon, the circle represents the back carbon

⑵ Torsional Strain: Energy difference due to the eclipsed conformation in conformational isomers

① Ethane: Eclipsed conformation is about 12 kJ/mol higher in energy compared to staggered conformation

○ Energy analysis

○ H-H staggered: 0 kJ/mol (reference)

○ H-H eclipsed: About 4 kJ/mol (1 kcal/mol) (torsional strain)

Figure 3. Newman Projection of Ethane

○ Stability of staggered conformation

○ Reason 1: Minimum electron-electron repulsion

○ Reason 2: Electrons in bonding (σ) MOs stabilize by interacting with antibonding (σ*) MOs

Figure 4. Staggered Conformation Stability in Ethane

② Propane: Eclipsed conformation is about 14 kJ/mol higher in energy compared to staggered conformation

○ Energy analysis

○ H-H staggered: 0 kJ/mol (reference)

○ H-H eclipsed: About 4 kJ/mol (1 kcal/mol) (torsional strain)

○ CH₃-H eclipsed: About 6 kJ/mol (torsional strain)

○ Eclipsed conformation introduces greater torsional strain due to steric hindrance from the bulky methyl group overlapping with hydrogen

③ Butane: Differentiated as anti-staggered (most stable), gauche staggered, eclipsed, etc.

Figure 5. Newman Projection of Butane

○ Newman projections for C1-C2, C3-C4 are possible, but C2-C3 provides a more detailed energy profile.

○ Energy analysis

○ H-H staggered: 0 kJ/mol (reference)

○ H-H eclipsed: About 4 kJ/mol (1 kcal/mol) (torsional strain)

○ CH₃-H eclipsed: About 6 kJ/mol (torsional strain)

○ CH₃-CH₃ fully eclipsed: About 13 kJ/mol (4 kcal/mol) (torsional strain + steric interaction)

○ H-CH₃ gauche: 0 kJ/mol

○ CH₃-CH₃ gauche: About 3.8 kJ/mol (0.9 kcal/mol) (steric interaction)

○ Instability in gauche

○ Slightly more unstable due to closer proximity of methyl groups compared to anti-staggered

○ This repulsion is called van der Waals strain or steric strain

○ Stability in anti

○ Reason 1: Minimum electron-electron repulsion

○ Reason 2: Electrons of methyl group stabilize by interacting with antibonding orbitals (σ*)

④ Rotation Barrier Energy: Energy difference between the highest and lowest energy conformations

○ Generally within the range of natural energy fluctuations

⑶ Ring Strain

① Ring Strain = Torsional Strain + Angle Strain + Steric Strain (van der Waals strain)

○ Steric hindrance (torsional strain): Structural instability due to van der Waals repulsion within a molecule

○ Angle strain: Difficult to achieve the ideal 109.5° bond angle in sp3 hybrids for cyclic compounds

○ Formation of sigma bonds in a non-linear fashion to achieve the ideal angle → instability

② Experimental Values for Ring Strain

○ cyclopropane ≫ cyclobutane > cyclopentane ≒ cycloheptane > cyclohexane

Table. 2. Comparison of Energy Levels in Cycloalkanes

③ Cyclopropane

○ Forms propane under H₂/Pt conditions due to its high instability

Figure 6. Cyclopentane and Newman Projection

④ Cyclobutane

○ Forms butane under H₂/Pt conditions due to its high instability.

○ Actual cyclobutane has a bent structure, resembling a butterfly shape.

○ The flip energy barrier of cyclobutane is about 1.4 kcal/mol, resulting in rapid ring flipping.

⑤ Cyclopentane

○ Adopts an envelope or half-chair conformation to reduce torsional strain.

○ Cyclopentane is considered a stable ring structure due to similar ring strain to cyclohexane.

○ Reason for the prevalence of 5- or 6-membered ring compounds in organic reactions.

⑥ Cyclohexane

○ No ring strain in cyclohexane → highly stable similar to hexane

○ Reason for the prevalence of 5- or 6-membered ring compounds in organic reactions

○ Reason: All bond angles in cyclohexane’s chair conformation are around 111°, matching the sp³ hybridization condition’s ideal angle

○ Chair, half-chair, twist boat, boat conformations exist: Energy sequence is chair < twist boat < boat < half-chair

Figure 7. Various Conformations of Cyclohexane

○ Boat-form cyclohexane has 1,3-diaxial interaction and 1,4-flagpole hydrogen interaction.

⑦ Cyclododecane ~ Cyclohexadecane

○ Virtually no ring strain

6. Cyclohexane

⑴ Drawing Cyclohexane

① Step 1: Identify two vertices and mark the most easily identifiable positions of hydrogen atoms above and below

② Step 2: Alternately mark the positions of the remaining four hydrogen atoms above and below

③ Step 3: From a specific carbon, draw parallel lines indicating the direction of hydrogen atoms in the opposite direction

④ Step 4: Finalization

Figure 8. How to Draw Cyclohexane

⑵ Newman Projection of Cyclohexane: The chair form of cyclohexane is always staggered.

① In cyclohexane boat and twist boat conformations, there is eclipsing, leading to instability due to torsional strain.

② 1,3-diaxial interaction

○ Newman projection can be drawn around any two adjacent carbons.

○ Named due to interaction occurring between carbons one carbon apart, like carbon 1 and carbon 3.

○ ■ and ■ are in a CH3-CH3 gauche staggered state and have an energy of about 3.8 kJ/mol.

Figure 9. Newman Projection of Cyclohexane

③ Application 1: 1,3-diaxial interaction in cis-decalin and trans-decalin

○ trans-decalin is about 3 × 3.8 kJ/mol more unstable than cis-decalin

○ Each ■, ■, and ■ represents a carbon atom, and the interaction of methyl groups around each carbon is indicated by dashed lines.

○ trans-decalin cannot undergo ring flip, while cis-decalin can.

Figure 10. Structure of Decalins and 1,3-diaxial Interaction

④ Application 2: 1,3-diaxial interaction in methylcyclohexane

○ When the methyl group is in the equatorial orientation (methyl equatorial, methyl horizontal): About 95%.

○ When the methyl group is in the axial orientation (methyl axial): About 5%. The 1,3-diaxial interaction of the methyl group exists.

Figure 11. Newman projection for 1,3-diaxial interaction of methylcyclohexane

Application 3. 1,3-diaxial interaction of 1,3-dimethylcyclohexane

Figure 12. 1,3-dimethylcyclohexane

○ 1,3-diaxial interaction between H and CH3 is 0.9 kcal/mol

○ 1,3-diaxial interaction between CH3 and CH3 is 3.6 kcal/mol

○ ΔG = ΔG(B) - ΔG(A) = (2 × 0.9 + 3.6) - 0 = 5.4 kcal/mol

○ A horizontally oriented substituent does not affect strain

○ B has one CH3-CH3 strain and two CH3-H strains compared to A

○ Conclusion: 1,3-dimethylcyclohexane is very stable in the cis form due to equatorial arrangement, making cis reaction dominant

⑥ Application 4. 1,3-diaxial interaction of tert-butylcyclohexane

Figure 13. tert-butylcyclohexane

○ 1,3-diaxial interaction between H and CH3 is 0.9 kcal/mol

○ 1,3-diaxial interaction between CH3 and CH3 is 3.6 kcal/mol

○ ΔG(C) = base + 4 × 0.9 (kcal/mol)

Figure 14. Process of calculating ΔG(C)

○ Because equatorial orientation of tert-butyl group is complex, deduction is made from trans-decalin form.

○ The cyclohexane in Figure C corresponds to the left cyclohexane in the figure above.

○ The red, blue, and green lines on the right all represent methyl groups: tert-butyl group in Figure C.

○ The red line ■ reflects H-CH₃ 1,3-diaxial interaction (2 × 0.9)

○ The blue line ■ reflects H-CH₃ 1,3-diaxial interaction (0.9)

○ The green line ■ reflects H-CH₃ 1,3-diaxial interaction (0.9)

○ ΔG(D) = base + 2 × 3.6 + 4 × 0.9 (kcal/mol)

Figure 15. Process of calculating ΔG(D)

○ Because axial orientation of tert-butyl group is more complex, deduction is made from cis-decalin form

○ The cyclohexane in Figure D corresponds to the right cyclohexane in the figure above

○ The red ■, blue ■, and green lines ■ at the bottom all represent methyl groups: tert-butyl group in Figure D

○ The red line ■ reflects CH3-CH3 1,3-diaxial interaction (2 × 3.6)

○ The blue line ■ reflects H-CH3 1,3-diaxial interaction (0.9)

○ The green line ■ reflects H-CH3 1,3-diaxial interaction (0.9)

○ ΔG = ΔG(D) - ΔG(C) = (2 × 3.6) - (2 × 0.9) = 5.4 kcal/mol

⑦ Application 5. trans-1,3-di-_tert_-butyl cyclohexane

○ When tert-butyl group is axial, energy instability increases, leading to a twist boat conformation

○ In this case, although ring strain increases, instability due to 1,3-diaxial interaction decreases significantly, resulting in overall stability

⑧ Application 6. tetrahydropyran-3-methanol

Figure 16. Stereoscopic structure of tetrahydropyran-3-methanol

○ In cases where a single substituent is in the axial position, it is generally unstable

○ However, in this case, the presence of intramolecular hydrogen bonding makes the axial position more stable

⑨ Application 7. Substituent-specific 1,3-diaxial interaction: The proportion equation that follows represents equatorial-axial ratio

○ -CN: 0.4 kJ/mol

○ -F: 0.5 kJ/mol

○ -Cl, -Br: 1.0 kJ/mol. 70: 30

○ -OH: 2.1 kJ/mol. 83: 17

○ -CO₂H: 2.9 kJ/mol.

○ -Me: 3.8 kJ/mol. 95: 5

○ -Et: 4.0 kJ/mol. 96: 4

○ - _i_Pr: 4.6 kJ/mol. 97: 3

○ -Ph: 6.2 kJ/mol

○ -t-Bu: 11.4 kJ/mol. 9999: 1

⑩ Application 8. Size of strain by substituent: Also known as A-value

○ -F: 0.26 kcal/mol

○ -Cl: 0.5 kcal/mol

○ -Br: 0.4 kcal/mol

○ -I: 0.47 kcal/mol

○ -CH₃: 1.8 kcal/mol

○ -Et: 1.9 kcal/mol

○ -_i_Pr: 2.0 kcal/mol

○ -t-Bu: 5.5 kcal/mol

○ -Ph: 3.0 kcal/mol

○ -C≡CH: 0.5 kcal/mol

○ -OH (protic): 0.9 kcal/mol

○ -OH (aprotic): 0.5 kcal/mol

○ -C≡N: 0.2 kcal/mol

○ -NO₂: 1.2 kcal/mol

⑪ Application 9. Substituent interaction with more than one group (general rule)

○ 1^st. Place the substituent with the largest 1,3-diaxial interaction horizontally

○ 2^nd. Consider cis and trans based on that carbon, draw the remaining substituents

○ Example: 1,3-dimethylcyclohexane is very stable in the cis form due to equatorial arrangement, making cis reaction dominant

⑫ Application 10. Anomeric Effect (Edward-Lemieux effect)

Figure 17. Anomeric Effect

○ Definition: The phenomenon where a substituent next to a heteroatom in a cyclohexane prefers an axial orientation

○ Epimer: Stereoisomers with multiple chiral centers differing in the stereostructure of only one

○ Anomer: Absolute arrangement difference of acetal or hemiacetal carbons in an epimer

○ Case 1: No heteroatom in cyclohexane: Substituent prefers equatorial orientation

○ Case 2: Substituent next to heteroatom -OH: axial orientation: equatorial orientation = 1: 2

○ Case 3: Substituent next to heteroatom -methoxy, -Cl, -acetyl: Prefers axial orientation

○ Difference between Case 2 and Case 3 corresponds to the difference between glucose and most glucose derivatives

○ Various mechanisms have been proposed for the specific mechanism

⑶ Ring Twist (cyclohexane ringflip, chair-chair interconversion)

① 10⁵ flips/s (25 °C)

② Switch horizontal to vertical, vertical to horizontal!

Figure 18. Ring flip of cyclohexane

Note that the upper and lower structures are not at the same energy level (because of steric hindrance)

⑷ Adamantane

Figure 19. Structure of adamantane

① Simplest 3D structure of diamond

② 7 carbons form 3 fused chair forms of cyclohexane

③ Expanded structures of adamantane: diamantane, triamantane

7. Alkane Reactions

⑴ Low reactivity

① All single bonds, non-polar → No acid-base reactions, no redox reactions

② Due to low reactivity, primarily used as organic solvents

③ Called paraffins due to low reactivity

⑵ Reaction 1. Combustion reaction

① Exothermic reaction

② Complete combustion: Products are water and carbon dioxide

③ Incomplete combustion: Products are carbon monoxide, formaldehyde, acetic acid

⑶ Reaction 2. Catalytic cracking

① Definition: Reaction where alkanes are heated, leading to bond cleavage or formation of multiple bonds

② Advantages: Economical method to synthesize large quantities of alkenes

③ Disadvantages: Not a great method as it forms various mixtures

⑷ Reaction 3. Reforming

① Straight chain alkanes transform into branched alkanes or aromatic compounds through reforming process

② Through cracking and reforming, gasoline yield becomes 47%: Natural gasoline ratio in petroleum is only 19%

⑸ Reaction 4. Radical Reaction

Input: 2018.12.27 16:12

Modified: 2024.03.29 07:49