A LevelChemistryOCRRevision Notes4. Core Organic ChemistryHaloalkanesHydrolysis of Primary Haloalkanes

Hydrolysis of Primary Haloalkanes (OCR A Level Chemistry A): Revision Note

Exam code: H432

Philippa Platt

Written by: Philippa Platt

Reviewed by: Caroline Carroll

Updated on

Hydrolysis of Primary Haloalkanes

Bond Enthalpy

  • The halogenoalkanes have different rates of substitution reactions

  • Since substitution reactions involve breaking the carbon-halogen bond the bond enthalpies can be used to explain their different reactivities

Haloalkane Bond Energy Table

Table of carbon-halogen bond enthalpies in kJ per mol: C-F 467 (strongest bond), C-Cl 346, C-Br 290, C-I 228 (weakest bond)

  • The table above shows that the C-I bond requires the least energy to break, and is therefore the weakest carbon-halogen bond

  • During substitution reactions the C-I bond will therefore heterolytically break as follows:

RCH2-I + OH-     →    RCH2-OH + I-

haloalkane           alcohol

  • The C-F bond, on the other hand, requires the most energy to break and is, therefore, the strongest carbon-halogen bond

  • Fluoroalkanes will therefore be less likely to undergo substitution reactions

Examiner Tips and Tricks

  • Bond energy / enthalpy values can vary in published literature despite theoretically being completed under the same conditions

  • Another reason for variation in bond energy / enthalpy values is due to the published values being the mean average of the specific bond from different substances

  • The key point is that you are able to use the data given to you and apply it in your answers

Aqueous silver nitrate

  • Reacting haloalkanes with aqueous silver nitrate solution will result in the formation of a precipitate

  • The rate of formation of these precipitates can also be used to determine the reactivity of the haloalkane

Haloalkane Precipitates Table

Table of precipitates formed with silver nitrate: chloroalkanes give white silver chloride, bromoalkanes give cream silver bromide, iodoalkanes give pale yellow silver iodide

  • The formation of the pale yellow silver iodide is the fastest (fastest nucleophilic substitution reaction) whereas the formation of the white silver chloride is the slowest (slowest nucleophilic substitution reaction); fluoroalkanes give no precipitate, as the C-F bond is too strong to be hydrolysed at a noticeable rate (and silver fluoride is soluble)

  • This confirms that fluoroalkanes are the least reactive and iodoalkanes are the most reactive haloalkanes

Arrow showing reactivity of haloalkanes increasing from fluoroalkanes (least reactive) through chloroalkanes and bromoalkanes to iodoalkanes (most reactive)

The trend in reactivity of haloalkanes

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Author
Reviewer
Philippa Platt

Author: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.