Chemical Bonding and Molecular Structure · Lesson 11 of 12
Hydrogen Bonding and Its Effects
“Explain how a polar hydrogen-containing bond can link molecules or connect suitable groups within the same molecule.”
• Explain why hydrogen attached to fluorine, oxygen, or nitrogen can participate in hydrogen bonding. • Identify the donor bond and the accepting electronegative atom in a hydrogen-bond diagram. • Distinguish intermolecular from intramolecular hydrogen bonding. • Relate hydrogen bonding to molecular association, physical state, and structure.
A polar bond can create an additional attraction
In water, an O–H covalent bond holds each hydrogen to oxygen within one molecule. Because oxygen attracts the shared electron density strongly, the hydrogen end is partially positive. That hydrogen can be attracted towards a suitable electron-rich atom nearby. This additional interaction is a hydrogen bond.
An attractive interaction involving hydrogen covalently attached to a strongly electronegative atom, commonly F, O, or N in this chapter, and a suitable electron-rich atom in the same or another molecule.
The hydrogen does not detach and travel freely between molecules in an ordinary hydrogen-bond diagram. It remains covalently attached to its donor atom. The second interaction is weaker than that covalent bond and is shown by a dotted line to distinguish the two links.
Why fluorine, oxygen, and nitrogen matter
Fluorine, oxygen, and nitrogen strongly attract shared electron density. When one is bonded to hydrogen, the resulting polar bond leaves a comparatively electron-poor hydrogen end. A nearby electronegative atom with available lone-pair density can act as an acceptor.
Hydrogen is also very small, so another atom can approach its electron-poor region closely. The combination of a strongly polar donor bond and a suitable nearby acceptor accounts for the important examples in this chapter. Merely having hydrogen somewhere in a molecular formula is not enough.
The notation X–H···Y is useful. X–H is the covalent donor bond, H···Y is the hydrogen-bond contact, and Y is the acceptor atom. The geometry must allow a close, favourable contact; the names of the elements alone do not establish that every conceivable pair will form a strong interaction.
Problem
In H–F···H–F···H–F, identify the covalent and hydrogen-bond contacts.
- 1.Each solid H–F link is a covalent bond within an HF molecule. Fluorine is relatively electron rich and hydrogen relatively electron poor.
- 2.Each dotted F···H contact joins the fluorine end of one molecule to the hydrogen end of the next.
- 3.The chain is an association of molecules. It does not mean that every solid and dotted segment is the same kind of chemical bond.
The covalent bond in a hydrogen molecule is an H–H bond. Hydrogen bonding is a different interaction involving a suitably polar donor bond and an electron-rich acceptor. The same word “hydrogen” does not make the mechanisms identical.
Intermolecular hydrogen bonding
An intermolecular hydrogen bond connects different molecules. The molecules may be of the same compound or different compounds, provided suitable donor and acceptor groups are available. This produces association beyond the covalent structure of each individual molecule.
HF, water, and alcohols provide the chapter’s examples. In water, an oxygen lone pair on one molecule can accept a contact from an O–H hydrogen on another. In an alcohol, the O–H group can provide the donor hydrogen and the oxygen can act as an acceptor. Their covalent molecular formulas do not change merely because such contacts form.
Problem
Describe one hydrogen bond between two water molecules.
- 1.Choose an O–H bond in the first molecule. Oxygen attracts its shared density, making H partially positive.
- 2.Choose a lone-pair region on the oxygen of the second molecule. This is the electron-rich acceptor region.
- 3.Draw O–H···O, keeping the first O–H line solid and the H···O contact dotted. Because two molecules are involved, the contact is intermolecular.
Intramolecular hydrogen bonding
An intramolecular hydrogen bond connects suitable groups within the same molecule. Molecular geometry must bring the donor hydrogen and an acceptor atom close together. These contacts can favour particular internal arrangements rather than directly linking separate molecules.
In o-nitrophenol, the hydroxyl group and nitro group occupy adjacent positions on the benzene ring. The hydroxyl hydrogen can interact with an oxygen of the nearby nitro group. The prefix ortho identifies this neighbouring placement, which makes the internal contact possible.
Problem
One drawing shows an O–H group attracted to a different molecule’s O. Another shows the hydroxyl H of o-nitrophenol attracted to its own nitro oxygen. Classify them.
- 1.First determine whether the donor and acceptor belong to one covalently connected molecular structure or two.
- 2.The first drawing joins different molecules, so it is intermolecular hydrogen bonding.
- 3.The o-nitrophenol contact remains inside one molecular structure, so it is intramolecular. The distinction concerns connectivity, not whether a dotted line happens to be drawn on the left or right of a page.
How hydrogen bonding affects substances
Intermolecular contacts create additional attractions that must be disrupted or rearranged when molecules separate or move. They can therefore influence physical properties and the arrangement of molecules in condensed phases. Intramolecular contacts can stabilise an internal shape and change which groups are readily available for intermolecular contacts.
The chapter describes hydrogen bonding as most extensive in the solid state and least extensive in the gas phase in its simple comparison. Read this as a statement about molecular proximity and association: closely packed molecules generally have more opportunity to maintain a network than widely separated gaseous molecules. The details depend on the substance and temperature, rather than a universal rule that every individual solid-state contact has identical strength.
A hydrogen bond is generally weaker than a covalent O–H, N–H, or F–H bond and is commonly stronger than a weak individual van der Waals contact in the examples being compared. These categories cover ranges of strengths. The important reasoning is that a collection of additional intermolecular contacts can have a substantial combined influence even when each is weaker than a covalent bond.
| Feature | Intermolecular | Intramolecular |
|---|---|---|
| Where the donor and acceptor lie | Different molecules | Within one molecule |
| Main structural effect | Links molecules into associations | Favours a suitable internal arrangement |
| Chapter examples | HF, water, alcohols | o-Nitrophenol |
| How it is drawn | Dotted H···acceptor contact between structures | Dotted contact within one connected structure |
Check your understanding
Identify the covalent donor bond before reading a dotted contact. Then check whether the acceptor is in the same molecule and whether the electron distribution supports the proposed attraction.
Quiz
In O–H···O, which part is the hydrogen bond?
Why is hydrogen in an O–H bond partially positive?
The hydrogen bond shown within o-nitrophenol is:
Which statement is sufficient by itself to prove hydrogen bonding?
Compared with the covalent donor bond, an ordinary hydrogen bond in these examples is generally:
Practice Problems
- Explain the charge distribution in an O–H or F–H bond and how it permits hydrogen bonding.
- Draw two water molecules with one hydrogen bond, labelling donor, acceptor, solid bond, and dotted contact.
- Show the intermolecular pattern of HF association and explain why the dotted links are not ordinary F–H covalent bonds.
- Use o-nitrophenol to explain the meaning of intramolecular hydrogen bonding and the importance of neighbouring groups.
- Explain why a substance can have weak individual intermolecular contacts but substantial overall attraction between many molecules.
- Compare the opportunities for molecular association in a condensed phase and a dilute gas, while avoiding a claim that phase alone fixes every bond strength.
- Distinguish the covalent H–H bond, the O–H bond in water, and the hydrogen bond between water molecules.
Key Takeaways
• A polar F–H, O–H, or N–H donor bond can support hydrogen bonding to a suitable electron-rich acceptor. • The donor covalent bond and the dotted hydrogen-bond contact are different interactions. • Intermolecular hydrogen bonds join molecules; intramolecular ones connect groups within one molecule. • Hydrogen bonding influences association, molecular arrangement, and physical properties. • A molecule’s geometry and available donor and acceptor groups matter.