CHEM2311 Lecture 4: Resonance Revisited, Sigma vs. Pi wrap up, Alkane Nomenclature

CHEM2311 Lecture 4: Resonance Revisited, Sigma vs. Pi wrap up, Alkane Nomenclature

Lecture Objectives

By the end of this class, students should be able to:

• Identify whether an atom with lone pairs is $sp^3$, $sp^2$, or $sp$ hybridized from a line structure, including if the lone pair is delocalized.
• Use orbitals to explain why single bonds can rotate but pi bonds cannot rotate. Use this to differentiate when two molecules are identical or non-identical.
• Identify alkene configuration as cis or trans.
• Differentiate between conformers, constitutional isomers, and configurational isomers.
• Draw multiple constitutional isomers for a provided structural formula.
• Explain why linear alkanes are said to be "fully saturated" and why cyclic alkanes are "unsaturated."
• Use IUPAC nomenclature rules to name linear, branched, and cyclic alkanes.
• Draw the line structure that corresponds to an IUPAC name.
• Use common names to name linear, branched, and cyclic alkanes.
• Draw the line structure that corresponds to a common name.
• Classify carbon atoms as primary, secondary, tertiary, or quaternary.

Hybridization and Orbital Energies

• Relative Orbital Energies: Atomic Orbitals (AOs) and Hybrid Orbitals (HOs) have specific energy levels.
  • $1s$ is lowest, followed by $2s$, then $2p$.
  • Hybrid orbitals like $sp^3$, $sp^2$, $sp$ exist between the energy levels of the original AOs.

Hybridization of Atoms with Lone Pairs

• To identify hybridization ($sp^3$, $sp^2$, or $sp$) of an atom with lone pairs from a line structure, one must consider if the lone pair is delocalized.
  • If a lone pair is involved in resonance (delocalized), the atom is generally $sp^2$ hybridized to allow the lone pair to reside in a p orbital that can overlap with other p orbitals.
  • If the lone pair is localized, the atom's hybridization is determined by the number of sigma bonds plus lone pairs (steric number).

Resonance and Delocalization

• Conditions for Delocalization: For electrons to be delocalized, they must reside in parallel p orbitals.

  • These p orbitals are most easily visualized in an edge-on perspective.
  • Example: 1,3-butadiene ($CH<em>2CHCHCH</em>2$) exhibits delocalization of pi electrons across all four carbon atoms via parallel p orbitals.
  • Example: Formic acid ($HCO_2H$) shows electron delocalization, particularly with the lone pairs on the oxygen atoms, which stabilizes the molecule.
    • The oxygen atom donating a lone pair into the pi system would be $sp^2$ hybridized.

• Result of Delocalization: Electron delocalization stabilizes the molecule.

Common Structural Patterns for Resonance

Three common patterns indicate the potential for resonance:

1. Alternating sigma and pi bonds.
2. A lone pair adjacent to a pi bond.
3. A carbocation adjacent to a pi bond.

Bond Properties: Sigma vs. Pi

Strength and Overlap

• Sigma ($\sigma$) bonds are stronger than pi ($\pi$) bonds.
  • Reason: Sigma bonds result from direct (head-on) overlap of atomic orbitals, leading to greater orbital overlap and thus stronger bonds.
  • Pi bonds result from indirect (side-on) overlap of p orbitals, which is a lesser degree of overlap, making them weaker.

Rotation

• Sigma bonds can rotate freely, whereas pi bonds cannot.
  • Reason for Sigma Bond Rotation: The direct overlap in a sigma bond is maintained during rotation, allowing free rotation around the bond axis.
  • Reason for Pi Bond Constraint: The side-on overlap of p orbitals forming a pi bond would be broken if rotation were to occur around the internuclear axis. Breaking this overlap requires significant energy, thus rotation is restricted.

Isomerism and Conformations

Conformers

• Definition: Conformers are identical molecules that result from the free rotation of single bonds.
• Characteristics: They have the same chemical formula, the same connectivity, and can be interconverted by bond rotation.
• Example: Butane can exist in various conformers (e.g., anti, gauche) due to rotation around the $C-C$ single bonds.

Isomers (Constitutional & Configurational)

• Definition: Isomers are two or more compounds with the same chemical formula but different connectivity or spatial arrangements that cannot be interconverted solely by rotation of a single bond.
• Characteristics: Isomers are considered different molecules.
  • Constitutional Isomers (Structural Isomers): Have the same chemical formula but a different connectivity of atoms.
  • Configurational Isomers: Have the same chemical formula and connectivity but differ in the spatial arrangement of atoms and cannot be interconverted by single bond rotation (e.g., cis/trans isomers).

Alkene Configuration (Cis/Trans)

• The restricted rotation of pi bonds leads to the possibility of cis/trans isomerism in alkenes.
• Cis: Substituents are on the same side of the double bond.
• Trans: Substituents are on opposite sides of the double bond.
• Example: Cis-2-butene and Trans-2-butene are configurational isomers because the double bond prevents free rotation, locking the substituents in distinct spatial arrangements.

Alkanes

Definition and Reactivity

• Definition: Alkanes are hydrocarbons that contain only single bonds.
• Reactivity: Without pi bonds or heteroatoms, alkanes are relatively unreactive.

Types of Alkanes

Alkanes come in different shapes:

• Linear Alkanes (or straight-chain alkanes)
• Branched Alkanes
• Cyclic Alkanes (or cycloalkanes)

Saturation in Alkanes

• Linear Alkanes are "fully saturated": This means they contain the maximum number of hydrogen atoms possible for a given number of carbon atoms.
  • General formula for an acyclic alkane: $C<em>nH</em>{2n+2}$.
  • Example: Hexane ($C<em>6H</em>{14}$) is fully saturated.
• Cyclic Alkanes are "unsaturated": Each ring in a cyclic alkane represents one "degree of unsaturation," meaning it has two fewer hydrogen atoms compared to its corresponding linear alkane.
  • General formula for a cycloalkane with one ring: $C<em>nH</em>{2n}$.
  • Example: Cyclohexane ($C<em>6H</em>{12}$) is unsaturated compared to hexane ($C<em>6H</em>{14}$).

Constitutional Isomers (Structural Isomers)

• Definition: Constitutional isomers (also known as structural isomers) contain the same chemical formula but a different connectivity of the atoms.
• Example: For the structural formula $C<em>6H</em>{14}$, multiple constitutional isomers can be drawn, such as n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane.

Alkane Nomenclature: Common Names

Linear (Normal) Alkanes

• Named by the number of carbons followed by the "-ane" suffix.
• The historical roots referring to the number of carbons (e.g., meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-) must be memorized.

Branched Alkanes and Alkyl Groups

• Branched alkanes: Have more than two carbons attached to one of the carbons in the alkyl chain (this definition is simplified here; typically, it means at least one carbon is bonded to three or four other carbons).
• Alkyl Groups: Alkane-based appendages on organic molecules; also called alkyl substituents.
  • Formed by removing one hydrogen from an alkane, replacing the "-ane" suffix with "-yl."
  • Example: Methane ($CH<em>4$) becomes Methyl ($-CH</em>3$); Ethane ($CH<em>3CH</em>3$) becomes Ethyl ($-CH<em>2CH</em>3$).

Common Alkyl Group Table

Carbon Classification

• The number of alkyl groups directly connected to a carbon atom determines its classification.
  

General Principles

• The International Union of Pure and Applied Chemistry (IUPAC) developed a detailed system for chemical nomenclature to provide unambiguous names for chemical compounds.

IUPAC Rules for Branched Alkanes

1. Identify the "main chain" or "base chain": This is the longest continuous chain of carbon atoms.
2. Identify and name the alkyl substituents: These are the branches coming off the main chain.
3. Number the main chain carbon atoms sequentially: Start at the end closest to a substituent to yield the lowest possible numbers for the substituents.
4. Assemble the full name: Identify the numerical position of each substituent and add the substituents to the front of the base chain. List substituents in alphabetical order. Use dashes to separate numbers from prefixes (e.g., 2-methyl).
   • Structure: [Position]-[Substituent name] + [Base name]
   • Example: 4-ethyl-3-methyloctane (For a chain of 8 carbons with ethyl at 4 and methyl at 3, alphabetizing 'e' before 'm').

Handling Multiple Identical Substituents

• If there is more than one of the same substituent, add a quantity prefix before the carbon numbers and the substituent name.
  • 2: di-
  • 3: tri-
  • 4: tetra-
  • 5: penta-
• Important: These quantity prefixes (di-, tri-, tetra-, etc.) are ignored when alphabetizing substituents.
  • Example: 2,4,4,5-tetramethyloctane (dimethyl- comes before ethyl- if comparing; here, 'methyl' is alphabetized, not 'tetramethyl').

Resolving Ambiguities in Numbering

• Scenario: If numbering from either end of the main chain results in the same number for the first substituent, compare the numbers for all substituents. Choose the numbering that gives the lowest numbers overall for all substituents.
  • Example: If one numbering gives 2,3,6-trimethylheptane and another gives 2,5,6-trimethylheptane, choose 2,3,6 as the first point of difference (3 vs. 5) is lower.

Resolving Ambiguities in Main Chain Selection

• Scenario: If there are two possible main chains of equal length, choose the chain that has the greatest number of substituents.
  • Principle: More substituents is generally preferred in IUPAC naming if the main chain length is equal.
  • Example: If a 7-carbon chain could be chosen in two ways, one giving 3-ethyl-2,4,5-trimethylheptane (4 substituents: 1 ethyl, 3 methyls) and another giving fewer, the one with more substituents is preferred.

Naming Cycloalkanes

• Root Determination: Whether the ring is treated as the root or as a substituent depends on the relative number of carbon atoms:
  • If the largest carbon ring has as many or more carbons than the longest continuous straight carbon chain, then the ring establishes the root as a cycloalkane.
    • Example: propylcyclooctane (cyclooctane is the root).
  • Otherwise, the longest continuous carbon chain establishes the root, and the cycloalkane is treated as a cycloalkyl substituent.
    • Example: 1-cyclopentylhexane (hexane is the root, cyclopentyl is the substituent).
• Numbering in Cycloalkanes: If there is only one substituent, no number is needed (the substituent is implied to be at carbon 1).
  • If there are multiple substituents, number the ring carbons to give the lowest possible numbers to the substituents, prioritizing alphabetically if a tie exists. If the main chain is an open chain and the ring is a substituent, number the open chain according to the standard rules.

Practice Problems and Solutions

Aktiv 1: Identify hybridization of indicated carbons.

• Solution requires visual, but context implies identifying hybridization in molecules with varying bonding and lone pairs.

Aktiv 2: Choose the correct common name for the alkyl substituent: -CH(CH3)2

• Solution: Isopropyl

Aktiv 3: Name the following using correct IUPAC nomenclature: $CH3CH2C(CH3)2CH_3$

• Solution: 2,2-dimethylbutane (The transcript shows CH3CH2CCH_3$$ with a methyl group coming off the second carbon from the right and another methyl attached to the same carbon, making it 2,2-dimethylbutane, not pentane or 3-methylbutane as some of the options suggest).

Aktiv 4: What is the correct (IUPAC) name for the compound below?

• Solution requires visual, but generally involves applying the 4 IUPAC rules.

Aktiv 5: Draw the skeletal structure for 2,3,4-trimethylheptane.

• Heptane: 7-carbon chain.
• Trimethyl: three methyl groups.
• 2,3,4: at carbons 2, 3, and 4.
• Skeletal structure would show a 7-carbon zig-zag chain with methyl branches at the 2nd, 3rd, and 4th carbons.

Aktiv 6: Provide the correct IUPAC name for the skeletal structure shown here.

• Solution requires visual. If it is 3-ethyl-2-methylpentane, this implies a 5-carbon main chain, with an ethyl group at position 3 and a methyl group at position 2, chosen because it has more substituents than another possible 5-carbon chain.