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The more electronegative atom of a bond is its δ− end: carbon is δ+ next to oxygen, nitrogen or a halogen, nearly neutral next to hydrogen, and δ− next to a metal, and the δ+ carbon is where nucleophiles attack.
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You will calculate electronegativity differences, say which end of a bond is δ+, rank bonds by polarity, and predict how a reversed carbon polarity changes reactivity.
Formal charge assumes every bond is shared evenly. Most bonds are not: one atom pulls harder. Polarity is the partial charge that results, and unlike formal charge it describes where the electron density really is.
Electronegativity is how strongly an atom pulls a shared pair; on the Pauling scale carbon is 2.55. A polar bond shares its pair unevenly; the atom with the larger share is δ− (partial negative), the other δ+. An electrophilic site is electron-poor; a nucleophilic one, electron-rich.
Pauling electronegativities of the atoms carbon usually bonds to:
| Atom | F | O | Cl | N | Br | C | H | Mg | Li |
|---|---|---|---|---|---|---|---|---|---|
| Value | 3.98 | 3.44 | 3.16 | 3.04 | 2.96 | 2.55 | 2.20 | 1.31 | 0.98 |
Anything to the left of carbon pulls the pair away from it: in C–O, C–N and C–Cl, carbon is δ+. Hydrogen and carbon itself are close enough that C–H and C–C are treated as nearly nonpolar. Metals are far to the right, so in C–Li and C–Mg the carbon is δ−.
The size of the difference says how polar: C–O at 0.89 is strongly polar, C–N at 0.49 moderately, C–H at 0.35 barely.
Another way: steps
For any bond:
A δ+ carbon is short of electrons and attracts electron-rich reagents: the carbon of a C=O or a C–Cl is where a nucleophile attacks (units 5 and 7). A δ− carbon, as in a Grignard reagent CH3MgBr, is itself electron-rich and seeks out δ+ carbon — which is how chemists make new carbon–carbon bonds. Reversing a carbon's polarity by bonding it to a metal is one of organic chemistry's most useful tricks.
A molecule's overall polarity is a separate question: in tetrachloromethane, CCl4, four polar C–Cl bonds point to the corners of a tetrahedron and cancel, so the molecule has no dipole, though every bond is polar.
Every bond to carbon is nonpolar. Only C–H and C–C are nearly so.
Carbon is always the positive end. Next to a metal it is the negative end.
The larger atom is the negative end. Electronegativity decides, not size.
A molecule with polar bonds is always polar. Bond dipoles can cancel.
Oxygen 3.44, carbon 2.55: a difference of 0.89.
Strongly polar.
Oxygen is δ−, carbon δ+.
Oxygen takes the larger share.
So the carbonyl carbon is the electrophilic site a nucleophile attacks.
Polarity points to reactivity.
Lithium 0.98, carbon 2.55. Which is δ−?
Carbon.
So is the CH3 carbon electrophilic or nucleophilic?
Nucleophilic: electron-rich, like a Grignard carbon.
Carbon's Pauling electronegativity is 2.55 and nitrogen's is $3.04$. What is the electronegativity difference across the C–N bond?
Answer:
In a C–O bond, oxygen has electronegativity $3.44$ and carbon $2.55$. Which statement describes the bond?
Match each bond to the partial charge on its carbon. (Electronegativities: C 2.55, H 2.20, O 3.44, Cl 3.16, Li 0.98.)
| δ+ | δ− | almost no partial charge | |
|---|---|---|---|
| the carbon of a C–O bond | |||
| the carbon of a C–Cl bond | |||
| the carbon of a C–Li bond | |||
| the carbon of a C–H bond |
Rank these bonds from most polar to least polar. (Electronegativities: C 2.55, H 2.20, N 3.04, O 3.44, F 3.98.)
Number the steps in order (write the number in the box):
A Grignard reagent, CH3MgBr, is added to propanone, (CH3)2C=O. Magnesium's electronegativity is 1.31 and carbon's 2.55. Which atom of the Grignard reagent seeks out the carbonyl carbon?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each bond in chloromethanol, ClCH2OH, name the atom at its δ+ end. (Electronegativities: H 2.20, C 2.55, O 3.44, Cl 3.16.)
| atom at the δ+ end | |
|---|---|
| C–Cl | |
| C–O | |
| O–H | |
| C–H |
You can read where a bond is short of electrons. Tell someone why the carbon of a Grignard reagent is δ−. Next: putting charge, resonance and polarity together to find a molecule's reactive site.
8. Your turn: methyllithium, CH3Li, step 3