CH 220: Nucleophilic Substitution

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40 Terms

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Starting material in this lab?
2-methyl-2-butanol (tertiary alcohol).
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Expected product?
2-chloro-2-methylbutane.
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Two requirements for substitution?
A nucleophile and a good leaving group.
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Nucleophile in this experiment?
Cl⁻ from HCl.
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Why is OH⁻ a problem?
It is a poor leaving group.
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What are the TWO roles of HCl?

  1. Provides Cl⁻

  2. Protonates OH to make H₂O a leaving group.

    • Converts poor leaving group OH⁻ into neutral H₂O.


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Why is the reaction SN1?

Tertiary alcohol forms a relatively stable carbocation.

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SN1 steps?

  1. Protonate the alcohol oxygen.

  2. Water leaves; carbocation forms.

    • (rate determining)

  3. Cl⁻ attacks the carbocation.


Protonate → Water leaves → Carbocation forms → Chloride attacks.


<ol><li><p>Protonate the alcohol oxygen.</p></li><li><p>Water leaves; carbocation forms.</p><ul><li><p>(rate determining)</p></li></ul></li><li><p>Cl⁻ attacks the carbocation.</p></li></ol><p></p><p>Protonate → Water leaves → Carbocation forms → Chloride attacks.</p><p></p>
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SN1 rate-determining step?

Leaving group departs to form the carbocation.

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SN1 vs SN2: intermediate / steps?

SN1 has a carbocation & is stepwise; SN2 has none & is concerted.

<p>SN1 has a carbocation &amp; is stepwise; SN2 has none &amp; is concerted.</p>
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Why is SN2 slow at tertiary carbon?
Steric hindrance blocks backside attack.
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fastest arrangement for SN1 vs SN2

SN1: tertiary alkyl halide (or resonance stabalized), weak nucleophile (h2o, OH)

SN2: primary alkyl halide (or methyl)

<p>SN1: tertiary alkyl halide (or resonance stabalized), weak nucleophile (h2o, OH)</p><p>SN2: primary alkyl halide (or methyl)</p>
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HCl volume and concentration?

12.5 mL of approximately 10.1 M HCl.
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Why use IR in this lab, what does it measure?

Check expected bonds and compare product spectrum with a reference.

Measures infrared absorption by molecular vibrations.

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Functional-group IR region?
About 4000–1250 cm⁻¹.
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Fingerprint IR region?
About 1250–400 cm⁻¹.
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Alkane C–H stretch location?
About 2850–3000 cm⁻¹.
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Alcohol O–H stretch location?
About 3200–3600 cm⁻¹.
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Carbonyl C=O stretch location?
About 1650–1750 cm⁻¹; typically strong.
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Which IR peak should disappear after reaction?
Alcohol O–H stretch (about 3200–3600 cm⁻¹).
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Which IR signal should remain?

Alkane C–H stretching around 3000 cm⁻¹.
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IR: Peak frequency vs intensity?

Frequency tells WHERE a peak occurs; intensity tells HOW STRONG it is.

<p>Frequency tells WHERE a peak occurs; intensity tells HOW STRONG it is.</p>
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Why does O–H peak width vary?
Different degrees of hydrogen bonding.
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What does a fingerprint match support?

Product identity compared with a known reference IR spectrum.

  • even molecules with similar functional groups can have different fingerprint-region patterns.


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Why can’t the lab IR reliably verify C–Cl?
Limited resolution near ~700 cm⁻¹.
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Why perform halide tests?
Support presence of alkyl halide and behavior of tertiary chloride.
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Which halide test follows SN2 vs SN1?

Sodium iodide (NaI) test vs Silver nitrate (AgNO₃) test.

<p>Sodium iodide (NaI) test vs Silver nitrate (AgNO₃) test.</p>
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Expected AgNO₃ result for tertiary chloride?

Precipitate forms very quickly.

<p>Precipitate forms very quickly.</p>
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Expected NaI result for tertiary chloride?
Very slow or no precipitate within five minutes.
Very slow or no precipitate within five minutes.
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Why do halide test rates differ?
Tertiary substrates favor SN1 and strongly hinder SN2.
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NaI test: what precipitates?

Sodium halide (NaX)

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AgNO₃ test: what precipitates?

Silver halide (AgX)

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What is the key HCl hazard?
Concentrated acid is corrosive; avoid exposure and spills.
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Four IR instrument rules?

Clean area; clean instrument; follow instructions; inspect spectrum.

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Nucleophile

Electron-rich species that attacks a carbon

  • ex. Cl-


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Why do we use HCl instead of NaCl?

  1. Supplies Cl⁻, the nucleophile.

  2. Supplies H⁺, which protonates –OH and converts it into H₂O, a good leaving group.


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Why does our experiment follow SN1?

2-methyl-2-butanol is a tertiary alcohol. Once protonated water leaves, a relatively stable tertiary carbocation can form, favoring the SN1 pathway.

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Overall reaction

2-methyl-2-butanol → 2-chloro-2-methylbutane

<p>2-methyl-2-butanol → 2-chloro-2-methylbutane</p>
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Wash versus extraction

Extraction

Wash

Transfers the desired compound into an extracting solvent

Transfers unwanted impurities into a washing solvent

Goal: collect the substance of interest

Goal: purify the product

Product moves into the extraction solvent

Product remains in its original phase


Our experiment uses a wash to remove excess HCl and other unwanted substances while keeping the organic product.

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IR observation from this lab

Most important observation: The alcohol O–H peak should disappear.

  • C-H present

  • OH & C=O absent

  • Fingerprint region simialr to known 2-chloro-2-methylbutane reference