CHEM-245 Exam 3 Review: Elimination Reactions, NMR & IR Spectroscopy, and Alcohol Reactions

Topic 7: Elimination Reactions

Alkene Stability

Alkene stability is influenced by both the substitution pattern and the geometric configuration (cis/trans) of the double bond.

• Substitution: Alkenes are classified as mono-, di-, tri-, or tetra-substituted based on the number of carbon groups attached to the double bond. More substituted alkenes are generally more stable due to hyperconjugation and alkyl group electron donation.

• Cis/Trans Isomerism: Trans alkenes are typically more stable than cis alkenes because of reduced steric repulsion between substituents.

• Example: 2-butene exists as both cis and trans isomers; the trans isomer is more stable.

Identifying β-Protons

β-protons are hydrogens located on the carbon atom adjacent to the carbon bearing the leaving group. Their removal is essential in elimination reactions.

• Key Point: Only β-protons can be abstracted during elimination to form the double bond.

E2 Reaction (Concerted Process)

The E2 (bimolecular elimination) reaction occurs in a single step, where the base removes a β-proton as the leaving group departs.

• Effect of Substrate: E2 is favored by strong bases and occurs most readily with secondary and tertiary alkyl halides.

• Regioselectivity: The Zaitsev product (more substituted alkene) is usually favored unless bulky bases are used.

• Stereoselectivity: Trans alkenes are preferred due to lower activation energy.

• Mechanism: Loss of leaving group and proton transfer occur simultaneously.

Equation:

$\mathrm{R{-}CH_2{-}CH(X){-}R'} + \mathrm{Base} \rightarrow \mathrm{R{-}CH=CH{-}R'} + \mathrm{Base{-}H} + X^{-}$

E1 Reaction (Stepwise Process)

The E1 (unimolecular elimination) reaction proceeds in two steps: first, the leaving group departs to form a carbocation, then a base removes a β-proton.

• Effect of Substrate: E1 is favored by tertiary substrates and weak bases.

• Regioselectivity: Zaitsev product is favored due to carbocation stability.

• Stereoselectivity: Trans alkenes are more stable and thus favored.

• Mechanism: Step 1: Loss of leaving group; Step 2: Proton transfer.

Equation:

$\mathrm{R{-}CH_2{-}CH(X){-}R'} \xrightarrow{\text{E1}} \mathrm{R{-}CH=CH{-}R'} + X^{-}$

Writing Complete Mechanisms

• E2: Show simultaneous loss of leaving group and β-proton abstraction.

• E1: Show stepwise loss of leaving group (carbocation formation) followed by β-proton abstraction.

Topic 8A: 1H NMR Spectroscopy

Theory Behind NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy uses an external magnetic field to probe the environment of hydrogen nuclei (protons) in organic molecules.

• Key Point: Protons absorb radiofrequency (RF) radiation and flip their nuclear spin when placed in a magnetic field.

• Application: NMR reveals the number and type of hydrogen atoms in a molecule.

Chemical Equivalency

Protons are chemically equivalent if they experience the same electronic environment. Non-equivalent protons give separate NMR signals.

• Key Point: Predict the number of unique 1H NMR signals by identifying equivalent and non-equivalent protons.

• Example: Ethanol (CH3CH2OH) has three unique proton environments.

Chemical Shift

Chemical shift (measured in ppm) indicates the electronic environment of protons. It is affected by nearby atoms and functional groups.

• Inductive Effect: Electronegative atoms deshield protons, shifting their signals downfield (higher ppm).

• Downfield vs. Upfield: Downfield = higher ppm, deshielded; Upfield = lower ppm, shielded.

• Use of Charts: Reference charts help predict expected chemical shifts for various functional groups.

Integration

Integration measures the area under each NMR signal, corresponding to the number of protons represented by that signal.

• Key Point: Integration ratios help determine the relative number of protons in each environment.

Multiplicity (Coupling of Protons)

Multiplicity arises from spin-spin coupling between neighboring protons, resulting in splitting patterns (singlet, doublet, triplet, etc.).

• Key Point: The n+1 rule: a proton with n neighboring protons will appear as an (n+1)-multiplet.

• Example: CH3 group next to CH2 appears as a triplet.

Predicting and Interpreting 1H NMR Spectra

• Prediction: Use chemical shift, integration, and multiplicity to predict the appearance of a spectrum for a given molecule.

• Interpretation: Deduce molecular structure from a given NMR spectrum by matching signals to possible environments.

Topic 8: IR Spectroscopy

Identification of Functional Groups

Infrared (IR) spectroscopy identifies functional groups based on their characteristic absorption frequencies.

• Key Point: Each functional group absorbs IR radiation at specific wavenumbers (cm-1).

• Example: O-H stretch appears around 3200-3600 cm-1.

General Shape of IR Absorbance Spectrum

An IR spectrum plots % transmittance versus wavenumber. Peaks correspond to bond vibrations.

• Three Main Regions:

  • Bonds to H (O-H, N-H, C-H): 2700-3700 cm-1

  • Triple Bonds (C≡C, C≡N): 2100-2300 cm-1

  • Double Bonds (C=O, C=C): 1600-1850 cm-1

• Wavenumber: Higher wavenumber = higher frequency, shorter wavelength.

Topic 9: Reactions of Alcohols

Classification of Alcohols

Alcohols are classified based on the number of carbon atoms attached to the carbon bearing the hydroxyl group.

• Methanol: Only one carbon (CH3OH).

• Primary (1°): One alkyl group attached.

• Secondary (2°): Two alkyl groups attached.

• Tertiary (3°): Three alkyl groups attached.

Converting -OH into a Better Leaving Group

Alcohols are poor leaving groups. Treatment with strong acids (e.g., HX) converts -OH into a better leaving group, facilitating substitution or elimination.

• Conversion to Alkyl Halides: Alcohol reacts with HX to form alkyl halide.

• Mechanism: Proton transfer, loss of leaving group, nucleophilic attack.

Equation:

$\mathrm{R{-}OH} + \mathrm{HX} \rightarrow \mathrm{R{-}X} + \mathrm{H_2O}$

Mechanisms: SN1 and SN2

• Secondary/Tertiary Alcohols: Undergo SN1 mechanism (carbocation intermediate).

• Primary Alcohols: Undergo SN2 mechanism (concerted).

• Stereochemical Outcome: SN1 leads to racemic mixture (50/50 R/S) due to planar carbocation.

Dehydration of Alcohols Using H2SO4

Alcohols can be dehydrated to form alkenes using sulfuric acid.

• Mechanism: Proton transfer, loss of leaving group, proton transfer.

• Secondary/Tertiary Alcohols: E1 mechanism (carbocation intermediate).

• Primary Alcohols: E2 mechanism (concerted).

• Regiochemical Outcome: Zaitsev rule applies; more substituted alkene is major product.

• Stereochemical Outcome: Trans alkenes are more stable than cis.

Equation:

$\mathrm{R{-}CH_2{-}CH(OH){-}R'} \xrightarrow{\text{H_2SO_4}} \mathrm{R{-}CH=CH{-}R'} + \mathrm{H_2O}$

Writing Full Mechanisms for Secondary/Tertiary Alcohols

• Key Point: Carbocation intermediate is formed, leading to possible rearrangements and racemization.

• Hint: Only secondary or tertiary alcohols will be asked for full mechanism (due to carbocation formation).