Hydrogen bonds form exclusively between different molecules.
Bonds within a single molecule (e.g., C-H or C-C) are NOT hydrogen bonds.
Example: Ethanol Molecules
Ethanol is an alcohol:
Has hydroxyl group (-OH) which participates in hydrogen bonding.
Example structure:
H connected to an O on one molecule can form a bond with an O on a second ethanol molecule.
Common Mistakes
Assuming H can bond with any other H:
Only H connected to O (like in -OH) can bond with other O's.
Misidentifying bonds:
C-H bonds cannot form hydrogen bonds.
H must be connected to O, F, or N to participate in hydrogen bonding.
Formation of Hydrogen Bonds
Covalent Bonds:
O and H form covalent bonds (electron sharing).
Electrons are shared UNEQUALLY:
O has a higher electronegativity and "hogs" the electrons.
This gives O a slight negative charge (δ-).
This leaves H with a slight positive charge (δ+).
Attraction:
The positive H will be attracted to the negative O in another molecule, forming the hydrogen bond.
Why C-H Cannot Form Hydrogen Bonds
In C-H, electrons are shared equally
No charges are developed.
Therefore, C-H cannot establish hydrogen bonds.
Atoms that Participate in Hydrogen Bonding
In addition to O, Fluorine (F) and Nitrogen (N) can also form hydrogen bonds:
They also have high electronegativity and can lead to partial charges.
F, O, N can bond with hydrogen - mnemonic: FON (Fluorine, Oxygen, Nitrogen).
Hydrogen Bonding Scenarios
Each hydrogen connected to F, O, or N can form a hydrogen bond with other electronegative atoms:
e.g., H connected to O can bond with H connected to F, and vice versa.
Conclusion
Key takeaway: Identify atoms and their bonds carefully to determine the possibility of hydrogen bonding. Only the appropriate H's (those connected to F, O, or N) can participate in forming hydrogen bonds across different molecules.