ochem week 9 part 2

H NMR Overview

• Introduction to H NMR:

  • Hydrogen NMR (H NMR) helps us observe electronic environments of various hydrogen atoms in a molecule.

  • Key factors include the number of electrons around central carbons, the effect of spinning, and splitting patterns based on adjacent hydrogen atoms.

Practice Analysis

• Spreadsheet Analysis:

  • Utilize chemical environments to identify plausible structures in provided examples.

  • Analyze unique carbons and their connectivity, for example, count peaks in the carbon NMR spectrum (i.e., 7 peaks = 7 unique carbons).

  • Evaluate candidate structures based on the number of unique carbons.

• Chemical Shift Values:

  • Not as effective for differentiating candidate structures; focus on unique hydrogen counts via integration instead.

Hydrogen Analysis

• Key Aspects of H NMR:

  • Chemical Shift:

    • Reflects the electronic environment of hydrogen; maximum observed shift is 3 for typical compounds.

  • Integration:

    • Indicates the number of protons (e.g., integrations of 2, 4, 6, 8 for total hydrogen counts).

    • Recognize that integration can help eliminate options by providing counts that don't match certain structures.

  • Splitting:

    • Represents interactions between hydrogen atoms. Use it as a last resort after analyzing integration and chemical shifts.

Reaction Diagrams and Kinetics

• Kinetics vs. Thermodynamics:

  • Thermodynamic properties assess stability (energy difference between start and end states).

  • Kinetic properties determine the rate of reactions and barriers (activation energies).

• Transition States:

  • At the peak of the energy diagram; represents unstable configurations during a reaction process where bonds are being made/broken.

  • Important to recognize the activation energy needed to reach this peak.

  • Intermediate:

    • A transient entity formed during the reaction that is more stable than transition states but less than products.

Application in Compounds

• Example: Ibuprofen:

  • NMR spectra are essential for determining structural information.

  • Utilize IR and H NMR data to confirm functional groups present based on peak characteristics.

  • Multiple potential structures can lead to homology between different chemical compounds (e.g. morphine vs fentanyl).

Additional Notes on Bonding and Stability**:

• Bond Strengths:

  • Single bond < Double bond < Triple bond in strength.

  • Stability of alkenes varies with substitution; more substituents lead to greater stability due to hyperconjugation.

• Key Concepts for Exams:

  • Understand bond lengths, strengths, reaction mechanisms, and energy diagrams.

  • Be prepared for theoretical questions that require integration of various concepts.

Techniques for Practical Application**:

• Integration of Reactions:

  • Analyze diagrams to understand both kinetics and thermodynamics cohesively.

  • Recognize how to estimate energy differences and predict products based on stability.

Study Recommendations**:

ochem week 9 part 2

no symmetry makes every carbon unique

integration levels are amount of hydrogens on main carbons

usually dip between 1500-2000 is a carbonyl, strong one might just be a ketone

1 degree= 2 hydrogens

look over this

CHAPTER 7

Bond Strengths Different bonds between different atoms have different strengths • Sigma bonds are stronger than pi bonds between the same 2 atoms A C=C double bond not 2x the strength of a C-C single bond The electronic environment around each atom impacts its bond strength Different double bonds will have different strengths, and different energies associated with being broken The energy difference between cis and trans is 2.8 kJ/mol

alkene stability

The specific nature of an alkene determines how strong it is • • This is often determined by substitution and measured using AHhydrogenation Smaller AHhydrogenation means a smaller difference in AHreactants and ΔΗ, products This can also be impacted by resonance, inductive effects, and sterics

free energy

Remember: AG AH - TAS Gibbs free energy determines whether or not a process or reaction will be spontaneous · Enthalpy determines if it will be exothermic or endothermic Entropy measures the disorder of the reaction or process Endergonic (+AG) and exergonic (-AG) are terms that describe the difference in the free energy of a reaction Systems at equilibrium exist where AG = 0

heat (thermodynamics) plus speed (kinetics) makes combustion

small hills of activation are faster than big hills

When plotting how a chemical reaction can occur, the energy associated with the reactants changing to products on an energy diagram. • The peak between 2 valleys represents a transition state The valley between 2 peaks represents an intermediate These diagrams can be plotted using AH or using AG When using AG, the term AG is used instead of Ea

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