Carbocations - Organic Chemistry Tutorials | CHEM 30A, Study notes of Organic Chemistry

Download Carbocations - Organic Chemistry Tutorials | CHEM 30A and more Study notes Organic Chemistry in PDF only on Docsity! Organic Chemistry Tutorials: Carbocations Page 1 Carbocations Based on a Fall 2001 Chemistry 30BH Honors project by Patricia Young Tutorial Contents A. Introduction B. Carbocation Classification C. Carbocation Stability D. Carbocation Formation E. Three Fates of a Carbocation F. Rearrangement Causing a Change in Ring Size Exercise Solutions A. Introduction A carbocation is molecule in which a carbon atom bears three bonds and a positive charge. Carbocations are generally unstable because they do not have eight electrons to satisfy the octet rule. H C H H open octet on carbon B. Carbocation Classification In order to understand carbocations, we need to learn some basic carbocation nomenclature. A primary carbocation is one in which there is one carbon group attached to the carbon bearing the positive charge. A secondary carbocation is one in which there are two carbons attached to the carbon bearing the positive charge. Likewise, a tertiary carbocation is one in which there are three carbons attached to the carbon bearing the positive charge. Methyl carbocations No C–C+ bonds H C H H CH3O C H H Primary (1o) carbocations One C–C+ bond CH3 C H H C H H Organic Chemistry Tutorials: Carbocations Page 2 Secondary (2o) carbocations Two C–C+ bonds CH3 C H CH3 Tertiary (3o) carbocations Three C–C+ bonds CH3 C CH3 CH3 Exercise 1 Exercise solutions can be found at the end of the tutorial. Label each carbocation as primary, secondary, or tertiary. (a) CH3 C CH2CH3 H (b) (c) O If the carbon bearing the positive charge is immediately adjacent to a carbon-carbon double bond, the carbocation is termed an allylic carbocation. The simplest case (all R = H) is called the allyl carbocation. R R R R R Generic allylic carbocation H2C CH2 The allyl carbocation If the carbon bearing the positive charge is immediately adjacent to a benzene ring, the carbocation is termed a benzylic carbocation. The simplest case is called the benzyl carbocation. R R Generic benzylic carbocation H H The benzyl carbocation If the carbon bearing the positive charge is part of an alkene, the carbocation is termed a vinylic carbocation. The simplest case is called the vinyl carbocation. Note that the carbon bearing the positive charge has two attachments and thus adopts sp hybridization and linear geometry. Organic Chemistry Tutorials: Carbocations Page 5 R RR R H Empty pz orbital of carbocation Adjacent C-H bond Hyperconjugative overlap This phenomenon is termed hyperconjugation. Since the overlap supplies electron density to the electron-deficient carbocation carbon, we predict that increasing the number of hyperconjugative interactions increases carbocation stability. Extending this idea, we predict that increasing the number of bonds adjacent to the carbocation by increasing the number of alkyl groups attached to the carbocation carbon results in an increase in carbocation stability. For example, a tertiary carbocation should be more stable than a secondary carbocation. This prediction is accurate. Our simple prediction suggests that any adjacent bonding electron pair will participate in carbocation hyperconjugation. However, only C–H and C–C bonds provide a significant level of increased stability. When considering the importance of hyperconjugation versus resonance as the more important stabilizing feature, resonance usually wins out. For example, a primary carbocation with resonance is more stable than a secondary carbocation without resonance. A secondary carbocation with resonance is usually more stable than a tertiary carbocation without resonance. The general rules for carbocation stability can be summarized as follows. (a) Increasing substitution increases stability. CH3+ (methyl; least stable) < RCH2+ (1o) < R2CH+ (2o) < R3C+ (3o; most stable) (b) Resonance is more important than substitution. For example, a secondary carbocation without resonance is generally less stable than a primary carbocation with resonance. Exercise 3 Rank the relative stability of the three carbocations in each set. (a) H2C C CH2, H3C C CH3 and (CH3)3C H H Organic Chemistry Tutorials: Carbocations Page 6 (b) In vinylic carbocations, the positive charge is assigned to a carbon with sp hybridization. How does this influence the carbocation’s stability? An sp orbital has more s character than an sp2 orbital. Electrons in an s orbital are closer to the nucleus and therefore more tightly held than electrons in a p orbital. This can be taken to mean that the electronegativity of carbon increases with increasing s character. Thus: sp carbon (most s character; most electronegative) > sp2 > sp3 (least s character; least electronegative). Electronegativity is a measure of electron attraction. So the stability of a cation is influenced by the electronegativity of the atom bearing the positive charge. The more electronegative the atom the less stable the cation. A vinylic carbocation carries the positive charge on an sp carbon, which is more electronegative than an sp2 carbon of an alkyl carbocation. Therefore a primary vinylic carbocation is less stable than a primary alkyl carbocation. Similar reasoning explains why an aryl carbocation is less stable than a typical secondary alkyl carbocation such as cyclohexyl carbocation. Because of their reduced stability, vinyl and aryl carbocations are not often encountered. D. Carbocation Formation Even though carbocations can be found in many organic reaction mechanisms, most carbocations are formed by one of only two basic mechanism steps: ionization of a carbon - leaving group bond or electrophilic addition to a π bond. Ionization of a Carbon - Leaving Group Bond. When a bond between a carbon atom and a leaving group ionizes, the leaving group accepts the pair of electrons that used to be shared in the covalent bond. This may leave the carbon atom with an open octet, resulting in a carbocation. The ionization is indicated with a curved arrow starting at the bond and pointing to the leaving group atom that accepts the electron pair. Better leaving groups or formation of a more stable carbocation result in lower activation energy and faster ionization. Carbon - leaving group bonding ionization is illustrated using an oxonium ion. (CH3)3C OH2 (CH3)3C + OH2 Organic Chemistry Tutorials: Carbocations Page 7 Carbocation formation by ionization of a leaving group occurs in many organic reactions such as the SN1 and E1 mechanisms. Not every leaving group ionization affords a carbocation, for example, the SN2 reaction. Electrophilic Addition to a π Bond. When an electrophile attacks a π bond, the π electron pair may form a new σ bond to the electron-deficient atom of the electrophile. (Not all additions to π bonds involve electrophiles or carbocations.) The other π bond carbon no longer shares the π electron pair, resulting in a carbocation. This addition is indicated with a curved arrow starting at the π bond and ending at the electron deficient atom of the electrophile. More powerful electrophiles or the formation of more stable carbocations result in lower activation energy and faster addition. Electrophilic addition to a π bond is illustrated by the reaction of HBr (an electrophile) with styrene (PhCH=CH2). Note that the more stable carbocation (secondary with resonance) is formed. This is a key mechanistic feature of Markovnikov’s Rule. H Br + Br Electrophilic addition to a π bond occurs in many reactions of alkenes, alkynes and benzene rings. Note every addition reaction forms a carbocation, for example, catalytic hydrogenation or ozonolysis. Exercise 4 Suggest the products of each mechanism step. (a) I (c) Cl HO H (b) H Br E. Three Fates of a Carbocation Now we consider how carbocations behave in reaction mechanisms. Generally speaking, carbocations are unstable due to their open octets and positive charges. Thus, their reactions will be strongly influenced by filling the octet of the carbon bearing the positive charge, or at least making this positive charge more stable. There are three common mechanism pathways (or fates) by which carbocations may achieve this stability. These Organic Chemistry Tutorials: Carbocations Page 10 This resistance to rearrangement is probably due to orbital alignment restrictions during the rearrangement transition state. E. Rearrangement Causing a Change in Ring Size Rearrangement may lead to a change in ring size. For example: 1,2-alkyl shift Primary Secondary The driving force for this rearrangement is formation of a more stable secondary carbocation from a less stable primary carbocation. What often puzzles students is how to draw the structure of the product. Here is a little trick that might help. Begin by redrawing the starting structure: Next, number the ring to keep track of the atoms, then make the bond changes suggested by the curved arrows, but leave the atoms in place. This may lead to a funky structure, but this will be fixed in the next step. In this case, the C1–C5 bond shifts, taking a pair of electrons from C1, leaving C1 with an open octet and a positive formal charge. The former carbocation carbon gained an electron pair, so its formal charge becomes one unit more negative (+1 to zero). The five-membered ring has expanded to a six-membered ring. 1 2 3 4 5 1 2 3 4 5 Now redraw the rearranged product to make it look better, using the numbering scheme to keep the substituent positions in order. Organic Chemistry Tutorials: Carbocations Page 11 1 2 3 4 5 redraw 1 23 4 5 Exercise 6 Draw the rearranged carbocations based on the given curved arrows. Indicate how the carbocation stability has increased by this rearrangement. (a) CH2 H3C (b) O H3C Exercise 7 Draw the products based on the curved arrows. Name the carbocation fate illustrated. (a) H HOCH3 (b) Cl (c) H Exercise 8 Provide a carbocation to complete each reaction. Draw the curved arrows. (a) ??? OH2H2O (b) ??? H2O + H3O Organic Chemistry Tutorials: Carbocations Page 12 (c) 1,2-alkyl shift ??? Exercise 9 Illustrate the three carbocation fates using any molecules you want. Use curved arrows and give the products. Exercise Solutions Exercise 1 The carbon-carbon bonds that determine the carbocation type are shown in bold. (a) CH3 C CH2CH3 H secondary (b) tertiary (c) O primary Exercise 2 (a) (b) OCH3 OCH3 OCH3 (c) CH2 CH2 CH2 CH2 CH2 (d) No other significant resonance contributors can be drawn for this carbocation. Exercise 3 Use the general carbocation stability rules: (i) Methyl (least stable) < 1o < 2o < 3o (most stable), and Organic Chemistry Tutorials: Carbocations Page 15 Exercise 7 (a) Lose proton, form π bond. H HOCH3 + H2OCH3 (b) Capture nucleophile. Cl Cl (c) Rearrange. H Exercise 8 (a) OH2OH2 (b) H OH2 + H3O (c) 1,2-alkyl shift Exercise 9 Any carbocation and any other reactants are acceptable as long as the carbocation fates are accurately illustrated. Capture nucleophile: C H H CH2 + HOCH3 C H H CH2 O H CH3 Organic Chemistry Tutorials: Carbocations Page 16 Lose proton, form π bond: C H H HOCH3 CH2 C H CH2 + H2OCH3 Rearrange: C H H CH2 C H CH3