Organic Chemistry – Basic Principles (Reaction Mechanism)

Fundamental Concepts In Organic Reaction Mechanism

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Understanding organic reaction mechanisms is essential for predicting the products of reactions and designing synthetic pathways. It involves studying how reactions occur at a molecular level.

Fission Of A Covalent Bond

Covalent Bond: A covalent bond is formed by the sharing of electrons between two atoms.

Homolytic Fission:

• Description: Cleavage of a covalent bond where each atom gets one of the shared electrons.
• Products: Forms neutral species called free radicals.
• Representation: Single-headed arrow ($\curvearrowright$) indicates movement of one electron.
• Conditions: Usually occurs in the gaseous phase or non-polar solvents with energy input like heat or UV light.
• Example: $CH_3-CH_3 \xrightarrow{UV \ light} CH_3\cdot + \cdot CH_3$ (Ethane breaks into two ethyl radicals)

Heterolytic Fission:

• Description: Cleavage of a covalent bond where one atom gets both of the shared electrons, and the other atom gets none.
• Products: Forms charged species: a carbocation (electron-deficient carbon with a positive charge) and a carbanion (electron-rich carbon with a negative charge), or a molecule with separated charges.
• Representation: Double-headed arrow ($\curvearrowright$) indicates movement of an electron pair.
• Conditions: Usually occurs in polar solvents.
• Example: $CH_3-Cl \rightarrow CH_3^+ + Cl^-$ (Polar solvent helps stabilize the ions).

Substrate And Reagent

Substrate: The molecule or ion upon which the reagent acts. It is the starting material in an organic reaction that undergoes transformation.

Reagent: The molecule or ion that attacks the substrate, bringing about the chemical change. Reagents are often responsible for initiating the reaction by interacting with the substrate.

Classification of Reagents: Reagents are classified based on the type of species they generate during heterolytic fission:

• Electrophiles ('electron-loving'): Species that are electron-deficient and seek electrons. They attack electron-rich centers. Usually have a positive charge or a partial positive charge, or have incomplete octets.

Examples: $H^+$, $NO_2^+$, $SO_3$, $R^+$, $R_3C^+$, $BF_3$, $AlCl_3$.

• Nucleophiles ('nucleus-loving'): Species that are electron-rich and seek electron-deficient centers. They donate an electron pair to form a new covalent bond. Usually have a negative charge or a lone pair of electrons.

Examples: $OH^-$, $CN^-$, $RS^-$, $R_3N:$, $H_2O:$, $NH_3:$, $RCOO^-$.

Electron Movement In Organic Reactions

Curved Arrows: The movement of electron pairs in organic reactions is depicted using curved arrows. The tail of the arrow starts at the source of the electron pair (a bond or a lone pair), and the head points to where the electron pair moves (an atom, a bond, or between two atoms).

• Single-headed arrow ($\curvearrowright$): Represents the movement of a single electron (in homolytic fission or radical reactions).
• Double-headed arrow ($\curvearrowright$): Represents the movement of an electron pair (in heterolytic fission or polar reactions).

Importance: Understanding electron movement is key to drawing reaction mechanisms and understanding how bonds are formed and broken.

Electron Displacement Effects In Covalent Bonds

The distribution of electron density in a covalent molecule can be influenced by the electronegativity differences between atoms and the arrangement of electrons. These effects are crucial for understanding reactivity.

Inductive Effect

Description: The inductive effect is the permanent displacement of $\sigma$-electrons along a carbon chain due to a difference in electronegativity between the bonded atoms. It operates through the sigma ($\sigma$) bond framework.

Types:

• +I Effect (Electron Donating): Occurs when an alkyl group or an atom/group more electropositive than carbon is attached to the chain. The group pushes electron density along the chain.

Alkyl groups ($CH_3-$, $C_2H_5-$ etc.) exhibit +I effect. The effect increases with the number of carbon atoms in the alkyl group and branching.

• -I Effect (Electron Withdrawing): Occurs when an atom or group more electronegative than carbon is attached to the chain. The group pulls electron density along the chain.

Examples: Halogens ($F > Cl > Br > I$), $-NO_2$, $-CN$, $-COOH$, $-OH$, $-NH_2$. The effect decreases with distance from the electronegative atom/group.

Nature: It is a permanent effect but weakens rapidly with distance.

Applications: Explains the acidity of carboxylic acids, basicity of amines, and stability of carbocations and carbanions.

Resonance Structure

Description: Resonance is a concept used to describe the delocalization of $\pi$ electrons (and sometimes lone pairs) in molecules where a single Lewis structure cannot adequately represent the bonding. Resonance structures are different hypothetical Lewis structures that can be drawn for a molecule or ion, differing only in the arrangement of $\pi$ electrons and lone pairs.

Conditions for Resonance:

• Presence of $\pi$ bonds or lone pairs adjacent to a $\pi$ bond or an empty p-orbital.
• Conjugation: Alternating single and multiple bonds, or lone pairs/charges adjacent to multiple bonds.

Resonance Hybrid: The actual structure of the molecule is a weighted average of all contributing resonance structures, called the resonance hybrid. The resonance hybrid is more stable than any of the individual contributing structures.

Representation: Resonance structures are connected by double-headed arrows ($\leftrightarrow$).

Example: Benzene ($C_6H_6$). The actual structure is a resonance hybrid of two Kekulé structures.

Resonance Energy: The difference in energy between the resonance hybrid and the most stable contributing structure is called resonance energy, which contributes to the stability of the molecule.

Resonance Effect

Description: The resonance effect is the delocalization of $\pi$ electrons or lone pairs through a conjugated system, which results in a partial charge distribution across different atoms. It is often represented by curved arrows showing electron movement.

Types:

• +R Effect (Electron Donating): When an atom or group donates electron density to the conjugated system through resonance (e.g., $-OH$, $-NH_2$, $-OR$ attached to an aromatic ring). The group has lone pairs or $\pi$ bonds that can delocalize into the system.
• -R Effect (Electron Withdrawing): When an atom or group withdraws electron density from the conjugated system through resonance (e.g., $-NO_2$, $-CN$, $-CHO$, $-COOH$ attached to an aromatic ring). The group has multiple bonds or empty p-orbitals that can pull electron density away.

Nature: It is a permanent effect and can influence reactivity, acidity, basicity, and stability.

Electromeric Effect (E Effect)

Description: The electromeric effect is a temporary effect where the $\pi$ electrons of a multiple bond (double or triple) are completely transferred to one of the atoms in the multiple bond in the presence of an attacking reagent.

Types:

• +E Effect: $\pi$ electrons move to an atom in the multiple bond towards the attacking reagent.
• -E Effect: $\pi$ electrons move to an atom in the multiple bond away from the attacking reagent.

Nature: It is a temporary effect and occurs only in the presence of an attacking reagent. It is a characteristic of multiple bonds.

Example: Addition of $H^+$ to ethene ($CH_2=CH_2$):

$$CH_2=CH_2 + H^+ \rightarrow [CH_3-\stackrel{+}{C}H_2 \leftrightarrow CH_3-CH_2^+]$$

(The $\pi$ electrons move towards one carbon, forming a carbocation on the other).

Hyperconjugation

Description: Hyperconjugation is the delocalization of $\sigma$ electrons of $C-H$ bonds adjacent to an $sp^2$ hybridized carbon atom (or other unsaturated system) by the overlap of the $\sigma$ orbital with the empty $p$ orbital or $\pi$ orbital of the unsaturated system.

Conditions: Requires $\alpha$-hydrogens (hydrogens attached to the carbon atom adjacent to the $sp^2$ or $sp$ hybridized carbon). It is sometimes called the 'no-bond resonance'.

Nature: It is a permanent effect.

Applications: Explains the stability of carbocations, alkenes, and alkyl-substituted benzene rings. The more $\alpha$-hydrogens, the greater the hyperconjugation, and hence greater stability.

Example: Stability of carbocations:

$CH_3^+ > CH_3CH_2^+ > (CH_3)_2CH^+ > (CH_3)_3C^+$ (This order is generally incorrect; it should be the opposite). The correct order of stability of carbocations is:

$(CH_3)_3C^+ > (CH_3)_2CH^+ > CH_3CH_2^+ > CH_3^+$. This is because tertiary carbocations have more $\alpha$-hydrogens (9) than secondary (6), primary (3), and methyl (0), leading to greater stabilization by hyperconjugation.

Types Of Organic Reactions And Mechanisms

Organic reactions can be broadly classified based on the way bonds are broken and formed:

1. Substitution Reactions:

• Description: A reaction in which an atom or group of atoms is replaced by another atom or group of atoms.
• Types:
• Free Radical Substitution: Involves free radicals (e.g., halogenation of alkanes).
• Nucleophilic Substitution: A nucleophile replaces another group (often a leaving group).
• Electrophilic Substitution: An electrophile replaces a group on an aromatic ring.

2. Addition Reactions:

• Description: Reactions where atoms are added across a multiple bond (double or triple bond). The $\pi$ bond is broken, and two new $\sigma$ bonds are formed.
• Types:
• Electrophilic Addition: Occurs with alkenes and alkynes (e.g., addition of $HBr$, $H_2O$).
• Nucleophilic Addition: Occurs with carbonyl compounds (aldehydes and ketones).
• Free Radical Addition: Addition across double/triple bonds initiated by free radicals.

3. Elimination Reactions:

• Description: Reactions in which atoms or groups are removed from adjacent carbon atoms, forming a multiple bond. It's the reverse of addition.
• Example: Dehydrohalogenation of alkyl halides to form alkenes.

$CH_3CH_2Br \xrightarrow{strong \ base} CH_2=CH_2 + HBr$

4. Rearrangement Reactions:

• Description: Reactions where the carbon skeleton of a molecule is rearranged, usually involving the migration of atoms or groups.
• Example: Carbocation rearrangements.

Reaction Mechanisms: Organic reaction mechanisms describe the step-by-step pathway of a reaction, showing the movement of electrons, the formation of intermediates (like carbocations, carbanions, free radicals), and the transition states.