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Mechanism of the addition of HX to alkynes involves the intermediacy of the vinylic carbocation

Mechanism of the addition of HX to alkynes involves the intermediacy of the vinylic carbocation. 1. Every Brønsted-Lowry acid has a conjugate base; every Brønsted-Lowry base has a conjugate acid. 2. Brønsted-Lowry acids and bases can be neutral species, cations, or anions.

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Mechanism of the addition of HX to alkynes involves the intermediacy of the vinylic carbocation

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  1. Mechanism of the addition of HX to alkynes involves the intermediacy of the vinylic carbocation

  2. 1. Every Brønsted-Lowry acid has a conjugate base; every Brønsted-Lowry base has a conjugate acid. 2. Brønsted-Lowry acids and bases can be neutral species, cations, or anions. • 3. A Brønsted-Lowry base must contain at least one lone pair of electrons to form a bond with H+, which has no electrons of its own. • 5. The stronger acid reacts with the stronger base to form a weaker acid and a weaker base. • 6. The stronger the acid, the weaker its conjugate base, and vice versa. • 7. Neither H+(aq) nor H3O+(aq) is a true representation of the aquated proton. Both are shorthand notations and as such are used interchangeably. However, H3O+(aq) is far preferable!

  3. Brønsted-Lowry Acid:Substance that can donate H+ Brønsted-Lowry Base:Substance that can accept H+ Chemical species whose formulas differ only by one proton are said to be conjugate acid–base pairs.

  4. Brønsted-Lowry Acid–Base Concepts

  5. Weak Acid Ionization Constants ACIDCONJ. BASE Ka pKa=-logKa HF HNO2 C9H8O4 (aspirin) HCO2H (formic) C6H8O6 (ascorbic) C6H5CO2H (benzoic) CH3CO2H (acetic) HCN C6H5OH (Phenol) F– NO2 – C9H7O4 – HCO2 – C6H7O6 – C6H5CO2 – CH3CO2 – CN – C6H5O – 7.1 x 10 –4 4.5 x 10 –4 3.0 x 10 –4 1.7 x 10 –4 8.0 x 10 –5 6.5 x 10 –5 1.8 x 10 –5 4.9 x 10 –10 1.3 x 10 –10 3.1 3.3 3.5 3.8 4.1 4.2 4.7 9.3 9.9

  6. Very Weak Acid Ionization Constants ACIDCONJ. BASE pKa CH3COCH2COCH3 CH3NO2 H2O C2H5OH CH3COCH3 RCCH RCH=CH2 CH3CH3 CH3COCH –COCH3 – CH2NO2 OH– C2H5O– CH3COCH2– RCC– RCH=CH– CH3CH2– 9.0 10.2 15.7 15.9 20 25 44 50

  7. Brønsted-Lowry Acid Strength 1. For binary acids in the same group acidity increases with increasingsize of A. 2. For binary acids in the same row acidity increases with increasing electronegativity of central atom in A. 3. In most cases acidity increases as the ability of the conjugate base to disperse negative charge increases. The lower the charge-to-radius ratio of the conjugate base the more stable it is and the stronger its conjugate acid.

  8. Brønsted-Lowry Acid Strength

  9. Brønsted-Lowry Acid Strength Oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity.

  10. Brønsted-Lowry Acid Strength Oxoacids with same central atom, acid strength increases with increasing size.

  11. ____

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