Chapter 2:Remembering How We Do It:Mechanisms 21 Bond→lone pair An example the tan oi ti ved ow begins at the boning par.The n ad of the is at the The ov charge),the products are+1-1=0. Figure 2-5: CH2CH2CHg CH2CH2CHg 30n0-0 +: lone par CH2CH3 Bond→bond An example of a bond-to-bond step is shown in Figure 2-6.The tail of the curved arrow begins at one of the bonding pairs of the double bond (the n-bond),while the head points to where the new bond will form. bon →◇⊙ A more common example of this process involves two arrows and the shifting of two electron pairs.An example of of th s pro S IS own in Figure 2-7.The tail bonding pairs of the rbond oints xygen to mov oxygen oxygen Figure 2-7 Two electron- pair ovements
Chapter 2: Remembering How We Do It: Mechanisms 21 Bond → lone pair An example of the bond–to– lone pair combination is shown in Figure 2-5. In this example, the tail of the curved arrow begins at the bonding pair. The head of the curved arrow is at the chlorine atom where it forms a lone pair. The overall charge doesn’t change. The original compound was neutral (0 charge), the products are +1 – 1 = 0. Figure 2-5: Bond–to– lone pair movement. CH3 C CH2CH2CH3 CH2CH3 Cl CH3 C CH2CH2CH3 CH2CH3 + Cl Bond → bond An example of a bond-to-bond step is shown in Figure 2-6. The tail of the curved arrow begins at one of the bonding pairs of the double bond (the π-bond), while the head points to where the new π-bond will form. Figure 2-6: Bondto-bond movement. A more common example of this process involves two arrows and the shifting of two electron pairs. An example of this process is shown in Figure 2-7. The tail of the curved arrow again begins at one of the bonding pairs of the π-bond, while the head points to where the new bond will form. This movement forces the bonding pair between the hydrogen and oxygen to move to the oxygen atom to create a lone pair on the oxygen atom. Figure 2-7: Two electronpair movements. H O H H H O H H 06_178157-ch02.indd 21 5/28/10 9:45 AM
22 Part I:Brushing Up on Important Organic Chemistry I Concepts When more than one curved arrow is present,they should all point in the same general direction and never toward each other or away from each other.However,curved single-headed arrows do not necessarily follow this rule. Lone pair→bond An example of the lone pair-to-bond step is shown in Figure 2-8.In this step the tail of the curved arrow begins at the lone pair.The head of the curved arrow is going to form the C-N bond.Notice that there's conservation of the positive charge.In any mechanism,the overall charge must remain the same. to-bond Combining the Basic Moves Acommon error in a mechanism is to attempt to do too much in a single 乙eean山reheYo c o much is having arrows po e step. site direcioainhenet step.but re e temptation tthese two steps The best way to see how these steps work together is with an Begin by examining the conversion of t-butylalcphol tot-butyl chlorde This ess has a 96 percent yield.(This is a good thing!The overall reaction is shown in Figure 2-9. igure 2 +H20
22 Part I: Brushing Up on Important Organic Chemistry I Concepts When more than one curved arrow is present, they should all point in the same general direction and never toward each other or away from each other. However, curved single-headed arrows do not necessarily follow this rule. Lone pair → bond An example of the lone pair–to–bond step is shown in Figure 2-8. In this step, the tail of the curved arrow begins at the lone pair. The head of the curved arrow is going to form the C-N bond. Notice that there’s conservation of the positive charge. In any mechanism, the overall charge must remain the same. Figure 2-8: Lone pair– to–bond movement. NH3 NH3 Combining the Basic Moves A common error in a mechanism is to attempt to do too much in a single mechanism step, and a sure sign that you’re trying to do too much is having arrows pointing in opposite directions in the step. You can have arrows pointing in one direction in a step and in the opposite direction in the next step, but resist the temptation to combine these two steps. The best way to see how these steps work together is with an example. Begin by examining the conversion of t-butyl alcohol to t-butyl chloride. This process has a 96 percent yield. (This is a good thing!) The overall reaction is shown in Figure 2-9. Figure 2-9: Conversion of t-butyl alcohol to t-butyl chloride. C CH3 CH3 CH3 OH HCl conc. C CH3 CH3 CH3 Cl + H2O 06_178157-ch02.indd 22 5/28/10 9:45 AM
Chapter 2:Remembering How We Do It:Mechanisms 23 t reactions involving nair from the o hvdrogen from the hydrochloric acid.(See Figure 2-10.The m ement of the oxygen lone pair to the hydrogen"pushes"the bonding pair from the H-Cl bond onto the chlorine.This is a lone pair-to-bond transfer,which induces a bond-to-lone pair transfer. Protonation is almost always the first step in mechanisms involving an acid. CH to-bond noveme Step 2:The presence of a po sitive charge on the oxyger atom is unstable oxygen n such vity.I nding pair f ng s amp d is ch fa atee thi T good leav eOH group is n This is a bond-to-lone air transfer Figure 2-11 Step 2 Slow novemen Step 3:Forming a carbocation is difficult;however,tertiary carbocations, such as this one,can form as intermediates,or species that exist for a short time during the reaction.(See Figure 2-12.)The positive charge on the carbon makes this a strong electrophile that seeks a lone pair.In the final step of this mechanism,the carbocation accepts a lone pair from the chloride ion generated in the first step.The transfer is lone pair to bond
Chapter 2: Remembering How We Do It: Mechanisms 23 Step 1: This reaction, like most reactions involving an acid, begins with protonation. In this case, a lone pair from the oxygen forms a bond to the hydrogen from the hydrochloric acid. (See Figure 2-10.) The movement of the oxygen lone pair to the hydrogen “pushes” the bonding pair from the H-Cl bond onto the chlorine. This is a lone pair–to–bond transfer, which induces a bond–to–lone pair transfer. Protonation is almost always the first step in mechanisms involving an acid. Figure 2-10: Step 1: Lone pair– to–bond movement. C CH3 CH3 CH3 OH C CH3 CH3 CH3 OH H H Cl + ClStep 2: The presence of a positive charge on the oxygen atom is unstable because the oxygen has such a high electronegativity. The bonding pair from the C-O bond moves to the positive charge on the oxygen to become a lone pair. (See Figure 2-11.) In this case, this is the rate-controlling step, which is why this is an example of an SN1 mechanism. The water molecule formed is a good leaving group, which facilitates this reaction. (The OH group is not a good leaving group.) This is a bond–to–lone pair transfer. Figure 2-11: Step 2: Bond–to– lone pair movement. C CH3 CH3 CH3 OH H Rate determining Slow C CH3 CH3 CH3 + H2O Step 3: Forming a carbocation is difficult; however, tertiary carbocations, such as this one, can form as intermediates, or species that exist for a short time during the reaction. (See Figure 2-12.) The positive charge on the carbon makes this a strong electrophile that seeks a lone pair. In the final step of this mechanism, the carbocation accepts a lone pair from the chloride ion generated in the first step. The transfer is lone pair to bond. 06_178157-ch02.indd 23 5/28/10 9:45 AM
24 Part l:Brushing Up on Important Organic Chemistry I Concepts Figure 2-12: one pair to-bonc Each step includes a conservation of charge.Conservation of charge is an important part of all mechanisms. Intermediates that exists for a short time ediate is often essential to understanding the mechanism the curved arrows belp you in drawing the intermediate.Because you can use curved arrows in only three ways (bond to lone pair,bond to bond,and lone pair to bond).you have limited options for drawing intermediates. In the next example,a nucleophile attacks a double bond.(See Figure 2-13.) cophile is the hydroxide ion.The proce the hydroxide ion attacking the carbon atom at one end of car n-carbo e ca Figure 2-13: Nucleophilic 0 attack o a double OH bond You need to be very careful to keep the formal charges correct.It may help to remember that charge will be conserved. Don't forget:The nucleophile is at the tail of the arrow and the electrophile is at the head of the arrow
24 Part I: Brushing Up on Important Organic Chemistry I Concepts Figure 2-12: Step 3: Lone pair– to–bond movement. Cl C CH3 CH3 CH3 C Cl CH3 CH3 CH3 Each step includes a conservation of charge. Conservation of charge is an important part of all mechanisms. Intermediates In the preceding mechanism, the carbocation was an intermediate (a species that exists for a short time during the reaction). The form of the intermediate is often essential to understanding the mechanism. The curved arrows help you in drawing the intermediate. Because you can use curved arrows in only three ways (bond to lone pair, bond to bond, and lone pair to bond), you have limited options for drawing intermediates. In the next example, a nucleophile attacks a double bond. (See Figure 2-13.) In this case, the nucleophile is the hydroxide ion. The process begins with the hydroxide ion attacking the carbon atom at one end of the carbon-carbon bond. This is a lone pair–to–bond step. Next, a pair from the π-bond shifts to form another π-bond on the other side of the carbon atom. This is a bond-tobond transfer. Finally, a bond–to–lone pair transfer takes place. Figure 2-13: Nucleophilic attack of a double bond. O OH O OH You need to be very careful to keep the formal charges correct. It may help to remember that charge will be conserved. Don’t forget: The nucleophile is at the tail of the arrow and the electrophile is at the head of the arrow. 06_178157-ch02.indd 24 5/28/10 9:45 AM
Chapter 2:Remembering How We Do It:Mechanisms 25 point than a hydrogen atom,it is acting like a nucleophile.For example both the methoxide ion(CH,O)and the t-butoxide ion((CH)O)are strong bases,but only the methoxide ion is a strong nucleophile.The t-butoxide ior is too big and bulky to attack efficiently.The effect of the bulky nature of the t-butoxide ion on its reactivity is an example of steric hindrance,which was discussed in your Organic I course (and,naturally,in Organic For Dummies). EMB A molecule with a lone pair of electrons to donate can behave as a nucleophile. The strength of the nucleophile(the nucleophilicity)is often related to Astrong is us ty a y of a engh】 wn y to the ability of When working with nucleophiles.keep a few additional points in mind: Nucleophiles that are negatively charged are stronger nucleophiles than neutral ones. Generally,nucleophilicity increases as you go down the periodic table. Nucleophilicity is decreased by steric hindrance. Keys to substitution and elimination mechanisms ourtypes of mechanisms are inherent to Organic Ch nisms (SI an N2)a se ith pes apply to Organic about th ese processes ecompl The S refers to a nucleophilic substitution process where me nucleophile attacks an electrophile and substitutes for some part of the electrophile.The E refers to an elimination process where the nucleophile attacks an electrophile and causes the elimination of something.The 1 and 2 refer to the order of the reaction.A 1(first order)means only one molecule determines the rate of the reaction,whereas a 2(second order)means that a combination of two molecules determines the rate of the reaction.In many cases,two or more of these mechanisms are competing and more than one product may result
Chapter 2: Remembering How We Do It: Mechanisms 25 Some materials may behave as either a base or a nucleophile. The hydroxide ion is an example. When the nucleophile attacks and removes a hydrogen ion, it is behaving as a base. When the nucleophile is attacking at some other point than a hydrogen atom, it is acting like a nucleophile. For example, both the methoxide ion (CH3O– ) and the t-butoxide ion ((CH3)3O– ) are strong bases, but only the methoxide ion is a strong nucleophile. The t-butoxide ion is too big and bulky to attack efficiently. The effect of the bulky nature of the t-butoxide ion on its reactivity is an example of steric hindrance, which was discussed in your Organic I course (and, naturally, in Organic I For Dummies). A molecule with a lone pair of electrons to donate can behave as a nucleophile. The strength of the nucleophile (the nucleophilicity) is often related to basicity. A strong nucleophile is usually a strong base and vice versa. But nucleophilicity and basicity aren’t the same. Basicity refers to the ability of a molecule to accept (bond with) an H+. The base strength is shown by its equilibrium constant. On the other hand, nucleophilicity refers to the ability of a lone pair of electrons to attack a carbon on an electrophile. When working with nucleophiles, keep a few additional points in mind: ✓ Nucleophiles that are negatively charged are stronger nucleophiles than neutral ones. ✓ Generally, nucleophilicity increases as you go down the periodic table. ✓ Nucleophilicity is decreased by steric hindrance. Keys to substitution and elimination mechanisms Four types of mechanisms are inherent to Organic Chemistry I. These are substitution reaction mechanisms (SN1 and SN2) and elimination reaction mechanisms (E1 and E2). The principles of these four types apply to Organic Chemistry II, and no review would be complete without a few reminders about these processes. The SN refers to a nucleophilic substitution process where some nucleophile attacks an electrophile and substitutes for some part of the electrophile. The E refers to an elimination process where the nucleophile attacks an electrophile and causes the elimination of something. The 1 and 2 refer to the order of the reaction. A 1 (first order) means only one molecule determines the rate of the reaction, whereas a 2 (second order) means that a combination of two molecules determines the rate of the reaction. In many cases, two or more of these mechanisms are competing and more than one product may result. 06_178157-ch02.indd 25 5/28/10 9:45 AM