Topic 7 · page 6 of 14

Predicting hydrogen bonding

To decide whether a substance has hydrogen bonds, check for an H atom bonded directly to N, O or F, and for a lone pair on N, O or F. Molecules with an O–H or N–H bond, such as alcohols and amines, pass both tests.

12 min read7.4

Read first: Hydrogen bonds

Two tests 7.4

Water, ammonia and hydrogen fluoride are the standard examples. The specification also asks you to predict hydrogen bonding in molecules like them. Use two tests.

MethodDoes this substance have hydrogen bonds?

  1. Is there an H atom bonded directly to N, O or F? Look for an O–H, N–H or H–F bond in the structure. An H on a carbon atom does not count.
  2. Is there a lone pair on a N, O or F atom in the neighbouring molecule? Every N, O or F atom has one or more lone pairs, so this is automatic when the neighbours are molecules of the same substance.

If both are true, molecules of the substance form hydrogen bonds with each other.

Test 1 does the work. If a molecule has an O–H, N–H or H–F bond, it passes. If it has N, O or F atoms but no H on them, it fails: the lone pairs are there, but there is no δ+ H to be attracted to them.

Molecule What the structure shows Hydrogen bonds with itself?
CH₃OH, methanol O–H bond; lone pairs on O Yes
C₂H₅OH, ethanol O–H bond; lone pairs on O Yes
CH₃NH₂, methylamine N–H bonds; lone pair on N Yes
H₂O₂, hydrogen peroxide O–H bonds; lone pairs on O Yes
CH₃CO₂H, ethanoic acid O–H bond (the H of the acid group) Yes
HCl no: Cl is not N, O or F No
H₂S no: S is not N, O or F No
CH₄ only C–H bonds No
CH₃F F is bonded to C, and there is no H on F; the H atoms are on C No
CH₃OCH₃, methoxymethane O has lone pairs, but no H is bonded to O No
CH₃COCH₃, propanone O has lone pairs, but no H is bonded to O No
(CH₃)₃N, trimethylamine N has a lone pair, but no H is bonded to N No

Drawing the ones you predict 7.4

When you are asked to show a hydrogen bond between two molecules of a substance, use the five steps from the last page. In molecules with a carbon chain, draw the chain as a group label such as C₂H₅. It does not take part in the hydrogen bond.

Two ethanol molecules joined by a hydrogen bond. The hydrogen of the O–H group of the left molecule points along a straight dashed line at a lone pair on the oxygen of the right molecule. Each oxygen also carries an ethyl group, C2H5.
Ethanol: the hydrogen of the O–H group bonds to a lone pair on the O of the next molecule.

Worked examplePredict and draw

Which of C₂H₅OH, CH₃OCH₃, HBr and CH₃NH₂ form hydrogen bonds with their own molecules? Draw one hydrogen bond for one of them.

  1. C₂H₅OH: O–H bond, so test 1 passes; O has lone pairs, so test 2 passes. Yes.
  2. CH₃OCH₃: both bonds to O are O–C. There is no O–H, so test 1 fails. No.
  3. HBr: Br is not N, O or F. No.
  4. CH₃NH₂: N–H bonds and a lone pair on N. Yes.
  5. Drawing (ethanol): as in the figure. δ+ on the H of the O–H, δ− on each O, lone pairs on each O, and a dashed line from the H of the first molecule straight to a lone pair on the O of the second, with O–H···O in a straight line.

Check yourself

  1. Which of CH₃OH, CH₃Cl, CH₃NH₂ and CH₄ form hydrogen bonds with molecules of the same substance? Give a reason for each.
  2. Draw two molecules of methanol, CH₃OH, with a hydrogen bond between them. Describe your drawing.
  3. Chlorine is very electronegative, so explain whether HCl forms hydrogen bonds.
  4. Fluoromethane, CH₃F, contains a fluorine atom with lone pairs. Explain why liquid CH₃F has no hydrogen bonds.
  5. Ethanol, C₂H₅OH, boils at 78 °C and methoxymethane, CH₃OCH₃, boils at −24 °C. Both are C₂H₆O. Which has hydrogen bonds, and why?
  6. Does propanone form hydrogen bonds with other propanone molecules? Explain.

Answers cover these until you have tried the questions

  1. CH₃OH: yes, it has an O–H bond and O has lone pairs. CH₃NH₂: yes, it has N–H bonds and N has a lone pair. CH₃Cl: no, there is no H bonded to N, O or F (the H atoms are on C, and Cl is not N, O or F). CH₄: no, only C–H bonds.
  2. Two CH₃OH molecules, each with two lone pairs on O. Mark δ− on each O and δ+ on the H of each O–H. Draw a dashed line from the H of the first molecule straight to a lone pair on the O of the second, with O–H···O in a straight line, and label it "hydrogen bond".
  3. No. Cl is not N, O or F. HCl has permanent dipole–permanent dipole interactions and London forces, but no hydrogen bonds.
  4. Hydrogen bonds need an H atom bonded to N, O or F. In CH₃F the F is bonded to carbon and has no H on it; the H atoms are bonded to carbon. There is a lone pair on F but no δ+ H to be attracted to it.
  5. Ethanol. It has an O–H bond, so it forms hydrogen bonds. Methoxymethane has no H bonded to O (both bonds to O are to carbon), so it has no hydrogen bonds. The molecules have the same number of electrons (26), so the hydrogen bonds explain why ethanol boils so much higher.
  6. No. The O has lone pairs, but there is no H bonded to O (the H atoms are bonded to carbon), so there is no δ+ H of the right kind.