Organic Chemistry I Chapter 6: Understanding Organic Reactions

What are addition reactions?

• reaction where we ONLY add atoms to organic molecules, we don’t lose any

What are elimination reactions?

• reaction where we ONLY lose atoms in organic molecules, we don’t gain any

What are substitution reactions?

• reactions where we gain an atom when we lose an atom, the atom we gain replaces the atom we lose

What is heterolytic bond breakage?

• Unequal breakage where one atom takes both electrons, both flow in one direction, goes to EN atom

What is homolytic bond breakage?

• Equal breakage where both electrons are divided evenly amongst the two atoms, two electrons flow in different directions

What kinds of bonds often break homolytically vs heterolytically? (2)

• Homolytically: non-polar or less polar

• Heterolytically: polar

How can we show when bonds are broken homolytically/heterolytically? (3)

• curved arrow pushing to show electron movement (how bonds break/form)

• Homolytic arrow: half arrow tip

• Heterolytic arrow: full arrow tip

What are carbon radicals?

• carbons that don’t have an octet, (< 8 electrons) and carbons that have an unpaired electron

What are carbocations?

• carbon with a positive charge when a bond breaks

What are carboanions?

• carbon with a negative charge when a bond breaks, (lone pair electrons can give a negative charge on a carbon)

Why are carbon radicals, carbocations, and carbanions unstable? (2)

• They are reactive intermediates, they are formed during a reaction and are not the final products but continue to react to yield the final products

• They violate the octet rule and put charges on the Carbons:

  • carbocations: 6 valence electrons w/ positive charge

  • carbon radicals: 7 valence electrons w/ no charge

  • carboanions: 8 valence electrons w/ negative charge

How can we calculate the heat of a reaction (ΔH°)? (3)

• Write balanced equation

• List all bonds broken and formed in rxn, then use Table of Bond Dissociation Energy to find BDE for all of them

• Add the BDE together to get the ΔH° for the reaction

What sign is given to the BDE for exothermic vs endothermic rxns? Connect forming/breaking bonds to being exothermic/endothermic? (2)

Exothermic:

• forming bonds (releasing energy)

• negative (gets rid of energy to form)

Endothermic:

• breaking bonds (absorbing energy)

• positive (need energy to break)

What is the equilibrium constant? (K_eq)

• K_eq = [P]/[R]

What is the change in Gibb’s Free Energy? (ΔG°) (2)

• Measurement of if a reaction may proceed from the reactant side to the product side

• Formula: ΔG° = ΔH° - TΔS°

How does ΔG° relate to K_eq?

• ΔG° = -RT㏑K_eq

  • R = gas constant

  • T = absolute temperature

What is the numerical value of K_eq and sign of ΔG° regarding this energy diagram?

K_eq > 1

ΔG° = negative (-)

What is the numerical value of K_eq and sign of ΔG° regarding this energy diagram?

K_eq = 1

ΔG° = 0

What is the numerical value of K_eq and sign of ΔG° regarding this energy diagram?

K_eq < 1

ΔG° = positive (+)

How do we know if something is product/reactant/neither favored when looking at the K_eq, ΔG°, and energy diagram? (3×3)

Product:

• ΔG°: -

• K_eq: >1

• energy diagram: reactant higher G° than product

Reactant:

• ΔG°: +

• K_eq: <1

• energy diagram: product higher G° than reactant

Neither:

• ΔG°: 0

• K_eq: = 1

• energy diagram: reactant and product have equal G° or at same level

What is ΔG vs ΔH vs ΔS?

ΔG: a measurement of if a reaction may proceed from the reactant side to the product side

ΔH: enthalpy change, heat involved in a reaction

ΔS: entropy change, how messy a system is (more entropy when molecules have more freedom)

Given an energy diagram, identify the following: a) x and y coordinates, b) energy level of reactant, intermediate, and product; c) transition states; d) activation energy, e) delta H or delta G for each step and the overall reaction; f) the rate determining (limiting) step in a stepwise reaction; g) assessment of reaction rate vs. reaction yield given an energy diagram. (16)

X and Y coordinates:

• Y-coordinate: (1) Free Energy

• X-coordinate: (2) Reaction Progress

Energy Level of Reactant, Intermediate, and Product:

• Reactant: (4)

• Intermediate: (9)

• Product: (5)

Transition States:

• on diagram: (8) peaks of every curve

• highest-energy, unstable configuration where bonds are partially broken and partially formed

Activation Energy:

• on diagram: (6) energy between starting step and peak

ΔH/ΔG for each step and overall reaction:

• each step: difference in energy from reactants of step to products of step

• overall reaction: (3) difference between final products and initial reactants

The rate determining (limiting) step in a stepwise reaction:

• on diagram: (7) step with highest activation energy

• this step has the largest energy barrier to cross, it dictates the overall speed (kinetics) of the entire sequence

Assessment of reaction rate:

• controlled by activation energy

• lower activation energy = faster rate

Assessment of Reaction Yield:

• controlled by ΔG/ΔH

• more negative Δ𝐺° favors a higher yield of products at equilibrium