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Part 2 CHM1C3 Organic Acids and Bases

Part 2 CHM1C3 Organic Acids and Bases. Content of Part 2. Definition of Bronsted acids and bases Definition of conjugate acids and bases K a p K a Typical p K a values Eplaining differences in acidity: Resonance Effects Eplaining differences in acidity: Inductive Effects. CHM1C3

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Part 2 CHM1C3 Organic Acids and Bases

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  1. Part 2 CHM1C3 Organic Acids and Bases

  2. Content of Part 2 Definition of Bronsted acids and bases Definition of conjugate acids and bases Ka pKa Typical pKa values Eplaining differences in acidity: Resonance Effects Eplaining differences in acidity: Inductive Effects

  3. CHM1C3 – Introduction to Chemical Reactivity of Organic Compounds– – Learning Objectives Part 2 – Organic Acids and Bases • After completing PART 2 of this course you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods. • (i) You should be able to show the equilibrium between an organic acid in water with its conjugate base and the hydroxonium ion. • (ii) You should know what Ka equals with respect to this equilbrium. • (iii) You should know the relationship between Ka and pKa. • (v) You should understand that the smaller the pKa or the more negative the pKa the stronger is the acid. • By consideration of resonance structures of structurally related organic acids you should be able to make an assessment of which structure is likely to be the most acidic. • (vii) By consideration of inductive effects in structurally related organic acids you should be able to make an assessment of which structure is likely to be the most acidic.

  4. proton acceptor proton acceptor proton donor proton donor Bronsted Acids and Bronsted Bases Bronsted Acid: A Bronsted acid (HA) is a compound which acts as a proton donor. Bronted Base: A Bronsted Base (B:) is a compound which acts as a proton acceptor. Bronsted Acid Bronsted Base Conjugate Base Conjugate Acid HA + B: A + BH

  5. Examples of Bronsted Acids and Bronsted Bases Conjugate Base Conjugate Acid Bronsted Acid Bronsted Base AH + B: A + BH CH3CO2H + CH3O CH3CO2 + CH3OH H3O + NH3: H2O: + NH4 H2SO4 + H2O: HSO4 + H3O

  6. HA + H2O: A + H3O Quantifying the Equilibrium: Ka The dissociation of an acid, HA, in water may be represented as The water is acting as the base. Furthermore, the water is acting as the solvent and is in huge excess. The degree of ionisation is quantified by the equilibrium constant…

  7. Large Small Small Large Values of Ka [1] Very strong acid Almost complete ionization large number Approaches infinity [2] Very weak acid No perceptible ionization small number Approaches zero

  8. A H O 3 - L o g K HA + H2O: A + H3O a 1 0 H A The pKa = -Log10Ka p = Very strong acid high ionization Very weak acid low ionization

  9. Some Heteroatom pKa Values i.e. atoms attached to acidic protons other than carbon STRONG ACID WEAK ACID

  10. Resonance Effects and Acidity

  11. Explaining the Differences in Acidity: Resonance Effects pKa 4.76 Lone pairs of electrons adjacent to double bonds are able to delocalise through a process referred to as resonance. This resonance process imparts stability on the anionic structure (see Part 1 of the course) Thus, carboxylate anion is more stable than the alkoxide anion. 15.5

  12. Explaining the Differences in Acidity: Resonance Effects pKa = 7.23 Stronger Acid 3-Nitrophenol pKa = 8.36 Weaker Acid

  13. 2-Nitrophenol Lone pair delocalised into p-system of the aromatic ring Lone pair delocalised into p-system of the nitro group

  14. 3-Nitrophenol Lone pair delocalised into p-system of the aromatic ring It is not possible for the lone pair to be positioned on the carbon atom adjacent to the nitrogen atom. Therefore, there is one less resonance structure in this case, and this anion is subsequently less stable, and more difficult to form from its protonated form.

  15. Explaining the Differences in Acidity: Resonance Effects pKa = 20 Weaker Acid pKa = 9 Stronger Acid

  16. An Enolate Dr Cox’s Lecture Course two resonance structures Less stable anion three resonance structures More stable anion

  17. Inductive Effects and Acidity

  18. Nature of anion is different Same Hydroxonium Ion: Protonated water Explaining the Differences in Acidity: Inductive Effects pKa 0.23 3.75 4.20 4.76 5.03

  19. This resonance is the same for all the acids above. Thus, the R groups are influencing the stability of the carboxylate anion R affects CO2-

  20. R = CF3 this is the strongest acid. CF3 = -I Inductive Group Therefore, Is the most stable anion. CF3 is a strong electron withdrawing group (-I group) and is pulling electron density away from the carboxylate, i.e. reducing the charge on the carboxylate, and thus stabilising it, in a relative sense. R = CH3 this is a weaker acid. CH3 = +I Inductive Group Therefore, Is a less stable anion. CH3 is an electron donating group (+I group) and is pushing extra electron density onto the carboxylate, i.e. increasing the charge on the carboxylate, and thus destabilising it, in a relative sense.

  21. Some Carbon Atom pKa Values i.e. carbon atoms attached to acidic protons VERY WEAK ACID NOT REALLY AN ACID!

  22. – Summary Sheet Part 2 – Organic Acids and Bases CHM1C3 – Introduction to Chemical Reactivity of Organic Compounds– A Bronsted acid is a compound which can donate a proton (H+). Once the proton has been donated the resulting structure is referred to as the conjugate base. A Bronsted base is a compound which can accept proton. Once the proton has been accepted the resulting structure is referred to as the conjugate acid. Any acid/base reaction is, in principle, an equilibrium process. The equilibrium can be quantified by considering the degree of ionisation of an acid dissolved in water, where the water acts as the Bronsted base. This quantification is referred to as the pKa and is equal to the –log Ka, where Ka is equal to the equilibrium concentration of the conjugate base multiplied by the equilibrium concentration of the hydroxonium ion divided by the equilibrium concentration of the Bronsted acid. Consideration of inductive and resonance effects on the conjugate base between structurally related compounds allows a qualitative assessment of the order of acidity. The more delocalised the lone pair of electrons (formed from deprotonation of the acid) the more stable the conjugate base. If the conjugate base is stabilised, the easier it will be formed, and thus the stronger the Bronsted acid will be.

  23. www for further pKa information http://classes.yale.edu/chem220a/studyaids/pKa.html http://www.chromatography.co.uk/TECHNIQS/Other/buffers.htm http://home.planet.nl/~skok/techniques/laboratory/pka_pkb.html http://www.wiu.edu/users/mftkv/Chem331/acidstrength.htm http://www.geocities.com/le_chatelier_uk/pka.html (interesting if you have audio!) http://www.chem.wisc.edu/areas/reich/pkatable/ (pKas in DMSO as solvent) http://www.agsci.ubc.ca/courses/fnh/410/protein/1_13.htm (pKas of aminoacids) http://classes.yale.edu/chem220a/studyaids/pKa.html http://www.chem.umd.edu/courses/chem231fribush/3-Chapter2-3.pdf

  24. Question 1: Acids and Bases Rationalise why acid A is a stronger acid than acid B. A, pKa = 11.2 B, pKa = 25

  25. Answer 1: Acids and Bases Rationalise why acid A is a stronger acid than acid B. A, pKa = 11.2 B, pKa = 25 Most stable anion, as charge more delocalised over three resonance structures, compared to 2 in the conjugate base of B. Therefore, A is most acidic

  26. Question 2: Acids and Bases A and B are two structurally related phenols. Identify the one which you think will be the most acidic. A B

  27. Answer 2: Acids and Bases A and B are two structurally related benzoic acids. Identify the one which you think will be the most acidic. Two establish which is the strongest acid we need to consider the conjugate base resonance structures. We will be able to establish which has the most resonance structures, and is therfore the most stable conjugate base and therefore the most easiest to form. Most Acidic A B 5 Resonance Structures 4 Resonance Structures

  28. Question 3: Acids and Bases A and B are two structurally related phenols. Identify the one which you think will be the most acidic. A B

  29. Answer 3: Acids and Bases A and B are two structurally related phenols. Identify the one which you think will be the most acidic. Two establish which is the strongest acid we need to consider the conjugate base resonance structures. We will be able to establish which has the most resonance structures, and is therefore the most stable conjugate base, and thus the easiest to form. Most Acidic A B 5 Resonance Structures 4 Resonance Structures

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