Nucleophilic Substitution

Nucleophilic Substitution

• A nucleophile is electron-rich, negatively charged, and seeks electrons.
• An electrophile is electron-poor, positively charged, and seeks electrons.
• High electronegativity leads to a more stable intermediate state, but not stable enough, as electrons are shared between carbon and oxygen.
• To restabilize, electrons on oxygen move between oxygen and carbon, breaking one bond and re-establishing a double bond, and forming a leaving group.

Cleavage Reactions

• Heterolytic cleavage forms a carbanion and a hydride ion, carrying the shared electrons.
• Homolytic cleavage breaks a bond, each compound taking one electron, forming highly reactive free radicals.

Oxidation-Reduction Reactions

• Oxidation involves electron transfer, resulting in the oxidized compound losing electrons.
• Reduction involves electron gain, resulting in the reduced compound gaining electrons.
• OIL RIG: Oxidation Is Loss, Reduction Is Gain.

Biological Oxidation

• Addition of oxygen, removal of electrons, or dehydrogenation (removing hydrogen, resulting in hydride ions).

Chemical Catalysis

• Enzyme: a protein that aids chemical reactions.
• Substrate: the molecule on which the enzyme acts.
• Active site: the enzyme's binding site, containing hydrophobic amino acids and polar amino acids facilitating acid-base or covalent catalysis.

Types of Catalysis

• Acid-base catalysis: donation or gain of protons, increasing the reaction rate through catalytic proton transfer.
• Covalent catalysis: covalent bond formation between the substrate and enzyme.

Acid-Base Catalysis

• Histidine's imidazole group, with a pKa close to the solution's pH, donates/accepts protons, facilitating the reaction.
• Base catalysis removes protons, indirectly participating in bond cleavage.
• Acid catalysis involves proton donation.

Covalent Catalysis

• Substrate binds covalently to the enzyme (E), enabling covalent catalysis.
• Example: Sucrose phosphorylase facilitates the reaction sucrose + Pi → glucose-1-P + fructose.

Learn about nucleophilic substitution, including the roles of nucleophiles and electrophiles, and the intermediate and transition states involved in the process.