1. STRUCTURES AND PHYSICAL PROPERTIES

Structures and Physical Properties — Simple Notes

Short intro: This page explains basic ideas about how molecules are built and how that affects things you see — like whether a substance dissolves in water, melts, or boils. I’ve kept the language very simple and used small examples.

1. Polarity of bonds (what makes a bond ‘uneven’)

When two atoms share electrons in a bond, one atom may pull the electrons closer. That side becomes a little negative (δ−), and the other side becomes a little positive (δ+). This is called a polar bond.

Simple rule: If two atoms differ a lot in how strongly they attract electrons (electronegativity), the bond is polar. Example order: F > O > Cl, N > Br > C, H.

2. Polarity of molecules (whole molecule)

A molecule may have many bonds. If the bond polarities add up to give a net separation of charge, the molecule is polar (has a dipole). If the bond dipoles cancel each other (because of symmetry), the molecule can be nonpolar even if it has polar bonds (example: CCl4).

Example: Water (H2O) is polar and has a dipole. Carbon dioxide (CO2) is linear, so its dipoles cancel and it is nonpolar.

3. Melting point & boiling point — very basic idea

Melting = solid → liquid. You must give energy to break the forces that hold particles in fixed places. Boiling = liquid → gas. You must give more energy to let particles escape to gas.

Ionic substances (like NaCl) have very strong attractions — therefore very high melting/boiling points. Non-ionic (covalent) substances usually have much lower melting/boiling points.

4. Intermolecular forces (why stuff sticks together)

  • Dipole–dipole: attraction between positive side of one polar molecule and negative side of another.
  • Hydrogen bonding: special, fairly strong attraction when H is attached to F, O, or N. (Explains why water has high boiling point.)
  • Van der Waals (London) forces: very weak attractions that arise from temporary dipoles — important for nonpolar molecules and increase with molecular size.

5. Solubility — the “like dissolves like” rule

Polar substances dissolve well in polar solvents (like water). Nonpolar substances dissolve well in nonpolar solvents (like hexane). Water is a very good solvent for ionic compounds because it is polar and can form hydrogen bonds.

Protic vs aprotic solvents (short)

Protic solvents (e.g., water, ethanol): they have H that can hydrogen-bond (O–H or N–H). They stabilize ions and nucleophiles by hydrogen bonding. Aprotic solvents (e.g., acetone, DMSO, DMF): do not hydrogen-bond to nucleophiles — they often make SN2 reactions faster.

6. Ion pairs

In concentrated solutions, positive and negative ions sometimes stick together temporarily — called ion pairs. This matters in some reaction behaviours and conductivity.

7. Acids and bases — very short definitions

Bronsted–Lowry: acid = proton (H+) donor; base = proton acceptor.
Lewis: acid = electron-pair acceptor; base = electron-pair donor.

8. Isomerism — same formula, different arrangement

Isomers are molecules with the same molecular formula but different structures or 3D arrangements.

Structural isomerism (simple types)

  • Chain isomerism: different carbon chain (straight or branched).
  • Position isomerism: same groups but in different positions on the chain.
  • Functional isomerism: different functional groups (e.g., alcohol vs ether).
  • Metamerism: different alkyl groups around the same functional group.

Stereoisomerism (3D)

  • Geometrical (cis–trans): groups arranged differently around a double bond.
  • Optical isomerism: mirror-image molecules (enantiomers) that rotate plane-polarized light (D and L or + and −).

Tips for remembering (quick)

  • If molecule is symmetrical → polarities cancel → often nonpolar.
  • H bonded to O, N, or F → hydrogen bond is possible → higher boiling/melting points.
  • Big molecules → stronger van der Waals → higher boiling points.
  • For solubility: think “like dissolves like”.

Detailed Notes:

For PDF style full-color notes, open the complete study material below:

PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/STRUCTURES AND PHYSICAL PROPERTIES

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