KOT122 Assignment 1 Sample: Resonance, NaH and Acidity

For information and reference only – not to be submitted as your own work.

This extract supports students working on the KOT122 organic chemistry tutorial (Assignment 1, 20 marks). The original questions rely on drawn structures, so the compounds for Questions 1 to 3 cannot be reproduced here. Instead, those parts show the method with clearly labelled illustrative examples, while Question 4 (avobenzone and dioxybenzone) is answered in full. You can browse more model answers in our answers library, and the companion guide is How to Answer Resonance, Acidity and NaH Questions in KOT122.

What the assignment asks

The tutorial also lists textbook exercises from Smith’s Organic Chemistry (7th edition) in Chapters 1 to 3. These chapters cover bonding, acids and bases, and functional groups with intermolecular forces, which is exactly the toolkit used below.

Question 1: resonance structures and the hybrid (6 marks)

Method. Resonance forms differ only in the position of electrons, never atoms. Move pi electrons or lone pairs towards the positive charge using curved arrows, keep the total charge constant, and check that no second-row atom exceeds an octet. Forms that keep every atom at an octet contribute most to the hybrid.

Illustrative example A (not your compound): the allyl cation, CH₂=CH–CH₂⁺. The pi bond donates towards the empty p orbital on the terminal carbon.

The hybrid shows a partial positive charge (δ+) on each terminal carbon and two C–C bonds of equal length, each with bond order about 1.5, drawn as dashed half-bonds. The charge is delocalised over two carbons, which is why the allyl cation is more stable than a simple primary carbocation.

Illustrative example B (not your compound): CH₃–O–CH₂⁺. A lone pair on oxygen can form a pi bond to carbon, giving CH₃–O⁺=CH₂. Here the second form is the major contributor, because every atom (including carbon) has a full octet. The hybrid places δ+ on both carbon and oxygen, with a C–O bond between single and double. Examiners reward the explicit statement of which form contributes most and why.

Question 2: products with NaH (6 marks)

Sodium hydride supplies the hydride ion (H^–), which in this context acts as a very strong base rather than a nucleophile. It removes the most acidic proton, and the by-product is hydrogen gas. The conjugate acid of hydride is H₂, with a pKa of about 35, so NaH can deprotonate any acid with a pKa well below that.

Three habits protect marks. First, identify the most acidic hydrogen in every compound before drawing anything. Second, write the balanced equation, including H₂. Third, name the conjugate base formed. If a compound has no proton more acidic than about 35, such as a simple alkane or an ether, state that no reaction occurs.

Need a chemistry tutor to check your own structures against the brief? Our homework helper service covers science problem sets, including chemistry.

Question 3: ranking acidity and the strongest conjugate base (5 marks)

Compare the stability of the conjugate bases. The more stable the anion, the stronger the parent acid. Work through four factors in order: the atom carrying the charge (element effect), resonance, inductive effects, and hybridisation.

Illustrative ranking (not your set). Using the pKa values from standard tables, the order of increasing acidity is: acetylene (pKa about 25) < ethanol (about 16) < phenol (about 10) < acetic acid (about 4.8). A lower pKa means a stronger acid.

The link between the two halves of the question is the key reasoning step: the strongest acid gives the weakest conjugate base, so the strongest conjugate base comes from the weakest acid. In this illustration that is acetylide.

Question 4: avobenzone versus dioxybenzone (3 marks)

Prediction. Dioxybenzone is more readily washed off when swimming.

Structures. Avobenzone (C₂₀H₂₂O₃) is a 1,3-diketone: 1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)propane-1,3-dione. Dioxybenzone (C₁₄H₁₂O₄) is a benzophenone, (2-hydroxy-4-methoxyphenyl)(2-hydroxyphenyl)methanone, carrying two phenolic OH groups and one OCH₃ group.

Explanation. Solubility follows the rule that like dissolves like. Water is a polar solvent that forms hydrogen bonds. The hydroxyl groups of dioxybenzone can donate hydrogen bonds to water, and its carbonyl and ether oxygens can accept them, so it is attracted to water more strongly. Avobenzone has no OH group that is free to hydrogen bond with water, and its large nonpolar tert-butylphenyl group increases the hydrophobic surface. Its predicted partition coefficient (XLogP about 4.8) reflects that lipophilicity. Avobenzone therefore stays in the oily sunscreen film on the skin, while dioxybenzone is washed away more readily by water.

Qualification worth one line. Both OH groups in dioxybenzone sit ortho to the carbonyl and can form intramolecular hydrogen bonds with it, and avobenzone exists largely as an intramolecularly hydrogen-bonded enol. This lowers the polarity of both molecules, and both are poorly soluble in water. The conclusion is relative: dioxybenzone has more hydrogen-bonding capacity and less hydrophobic bulk, so it is the more water-washable of the two.

If you are stuck on a chemistry tutorial and want the working checked, a quick look at our assignment review service is a sensible next step.

How this extract earns marks

About the expert

For science questions, the closest specialist on our expert panel is Harith R., who holds an MSc in Medical-Surgical and Adult Health from Monash University Malaysia. His listed specialisms are pharmacology, adult nursing care and APA 7 referencing.

Ready to hand over a full tutorial set? You can place your assignment request here, or read what other students say on our reviews page.

References

Clayden, J., Greeves, N., & Warren, S. (2012). Organic chemistry (2nd ed.). Oxford University Press.

Department of Chemistry, Indiana University. (n.d.). Approximate pKa chart of the functional groups: Values to know. https://www.chem.indiana.edu/wp-content/uploads/2018/03/pka-chart.pdf

National Center for Biotechnology Information. (n.d.). PubChem compound summary for CID 51040, Avobenzone. U.S. National Library of Medicine. https://pubchem.ncbi.nlm.nih.gov/compound/Avobenzone

National Center for Biotechnology Information. (n.d.). PubChem compound summary: Dioxybenzone. U.S. National Library of Medicine. https://pubchem.ncbi.nlm.nih.gov/compound/Dioxybenzone

Smith, J. G. (2023). Organic chemistry (7th ed.). McGraw Hill.

University of California, Los Angeles, Department of Chemistry and Biochemistry. (n.d.). Sodium hydride. In Illustrated glossary of organic chemistry. https://www.chem.ucla.edu/~harding/IGOC/S/sodium_hydride.html

University of Massachusetts Amherst. (n.d.). pKa table for organic and inorganic Brønsted acids. https://owl.umass.edu/departments/OrganicChemistry/appendix/pKaTable.html