Tertiary carbocations are stabilized by hyperconjugation and inductive effects
the verdict
INSUFFICIENT LEANING
refutedsupported
the weight of evidence
7 sources for · 0 against
The retrieved reference sources establish that tertiary carbocations are stabilized by hyperconjugation; however, the provided evidence text does not comprehensively cover stabilization via inductive effects.
In 1962, Olah directly observed the tert-butyl carbocation by nuclear magnetic resonance as a stable species on dissolving tert-butyl fluoride in magic acid. The NMR of the norbornyl cation was first reported by Schleyer et al.[8] and it was shown to undergo proton-scrambling over a barrier by Saunders et al.[9]
Properties
In organic chemistry, a carbocation is often the target of nucleophilic attack by nucleophiles like hydroxyl (OH−) ions or halogen ions. Carbocations are classified as primary, secondary, or tertiary depending on the number of carbon atoms bonded to the ionized carbon. Primary carbocations have one or zero carbons attached to the ionized carbon, secondary carbocations have two carbons attached to the ionized carbon, and tertiary carbocations have three carbons attached to the ionized carbon. Stability of the carbocation increases with the number of alkyl groups bonded to the charge-bearing carbon. Tertiary carbocations are more stable (and form more readily) than secondary carbocations; primary carbocations are highly unstable because, while ionized higher-order carbons are stabilized by hyperconjugation, unsubstituted (primary) carbons are not.
Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further classified in two main categories according to the coordination number of the charged carbon: three in the carbenium ions and five in the carbonium ions. Among the simplest carbocations are the methenium CH+3 (a carbenium ion), methanium CH+5 (a carbonium ion), acylium ions RCO+, and vinyl C2H+3 cations.
Until the early 1970s, carbocations were called carbonium ions. This nomenclature was proposed by G. A. Olah. Carbonium ions, as originally defined by Olah, are characterized by a three-center two-electron delocalized bonding scheme and are essentially synonymous with so-called 'non-classical carbocations', which are carbocations that contain bridging C–C or C–H σ-bonds. However, others have more narrowly defined the term 'carbonium ion' as formally protonated or alkylated alkanes (CR+5, where R is H or alkyl), to the exclusion of non-classical carbocations like the 2-norbornyl cation.
Carbonium ions can be thought of as protonated or alkylated alkanes, bearing the general formula CR5+ (R = alkyl or H). A typical example is the methanium ion, CH5+, which is formed by protonation of methane using a superacid. By necessity of having five bonds on carbon but only four valence electron pairs available for bonding, they feature delocalized 3c-2e σ bonding and are thus regarded as type of non-classical carbocation. Like carbenium ions, carbonium ions are often invoked as intermediates in the upgrading of hydrocarbons in refineries. They are generally fleeting intermediates with fluxional structures that are challenging to observe and interpret spectroscopically. They can undergo decomposition by expulsion of a proton or alkyl group, or by loss of H2 to give a carbenium ion.
Carbocations are reactive intermediates in many organic reactions. This idea, first proposed by Julius Stieglitz in 1899, was further developed by Hans Meerwein in his 1922 study of the Wagner–Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction, the E1 reaction, and in rearrangement reactions such as the Whitmore 1,2 shift. The chemical establishment was reluctant to accept the notion of a carbocation and for a long time the Journal of the American Chemical Society refused articles that mentioned them.
An NMR spectrum of a carbocation was first reported by Doering et al. in 1958. It was the heptamethylbenzenium ion, made by treating hexamethylbenzene with methyl chloride and aluminium chloride. The stable 7-norbornadienyl cation was prepared by Story et al. in 1960 by reacting norbornadienyl chloride with silver tetrafluoroborate in sulfur dioxide at −80 °C. The NMR spectrum established that it was non-classically bridged (the first stable non-classical ion observed).
In 1962, Olah directly observed the tert-butyl carbocation by nuclear magnetic resonance as a stable species on dissolving tert-butyl fluoride in magic acid. The NMR spectrum of the norbornyl cation was reported by Schleyer et al. It was shown to rapidly undergo proton-scrambling.
HF is typically not used because of its instability and its fast reactivity rate. Below is an example of a reaction between an alcohol and hydrogen chloride:
GREEN (Cl) = nucleophile BLUE (OH) = leaving group ORANGE (H) = hydride shift proton RED(H) = remaining proton
The alcohol portion (-OH) has been substituted with the nucleophilic Cl atom. However, it is not a direct substitution of the OH atom as seen in SN2 reactions. In this SN1 reaction, we see that the leaving group, -OH, forms a carbocation on Carbon #3 after receiving a proton from the nucleophile to produce an alkyloxonium ion. Before the Cl atom attacks, the hydrogen atom attached to the Carbon atom directly adjacent to the original Carbon (preferably the more stable Carbon), Carbon #2, can undergo hydride shift. The hydrogen and the carbocation formally switch positions. The Cl atom can now attack the carbocation, in which it forms the more stable structure because of hyperconjugation. The carbocation, in this case, is most stable because it attaches to the tertiary carbon (being attached to 3 different carbons). However, we can still see small amounts of the minor, unstable product.
Carbocation Rearrangements - Chemistry LibreTexts Skip to main content This action is not available.
Below is an example of a reaction between an alcohol and hydrogen chloride: GREEN (Cl) = nucleophile BLUE (OH) = leaving group ORANGE (H) = hydride shift proton RED(H) = remaining proton The alcohol portion (-OH) has been substituted with the nucleophilic Cl atom. However, it is not a direct substitution of the OH atom as seen in S N 2 reactions. In this S N 1 reaction, we see that the leaving group, -OH, forms a carbocation on Carbon #3 after receiving a proton from the nucleophile to produce an alkyloxonium ion.
Before the Cl atom attacks, the hydrogen atom attached to the Carbon atom directly adjacent to the original Carbon (preferably the more stable Carbon), Carbon #2, can undergo hydride shift. The hydrogen and the carbocation formally switch positions. The Cl atom can now attack the carbocation, in which it forms the more stable structure because of hyperconjugation. The carbocation, in this case, is most stable because it attaches to the tertiary carbon (being attached to 3 different carbons). However, we can still see small amounts of the minor, unstable product. The mechanism for hydride shift occurs in multiple steps that includes various intermediates and transition states.
Below is the mechanism for the given reaction above: Hydration of Alkenes: Hydride Shift In a more complex case, when alkenes undergo hydration, we also observe hydride shift. Below is the reaction of 3-methyl-1-butene with H 3 O + that furnishes to make 2-methyl-2-butanol: Once again, we see multiple products. In this case, however, we see two minor products and one major product. We observe the major product because the -OH substitutent is attached to the more substituted carbon. When the reactant undergoes hydration, the proton attaches to carbon #2. The carbocation is therefore on carbon #2.
Hydride shift now occurs when the hydrogen on the adjacent carbon formally switch places with the carbocation. The carbocation is now ready to be attacked by H 2 O to furnish an alkyloxonium ion because of stability and hyperconjugation. The final step can be observed by another water molecule attacking the proton on the alkyloxonium ion to furnish an alcohol. We see this mechanism below: Alkyl Shift Not all carbocations have suitable hydrogen atoms (either secondary or tertiary) that are on adjacent carbon atoms available for rearrangement. In this case, the reaction can undergo a different mode of rearrangement known as alkyl shift (or alkyl group migration).
Alkyl Shift acts very similarily to that of hydride shift. Instead of the proton (H) that shifts with the nucleophile, we see an alkyl group that shifts with the nucleophile instead. The shifting group carries its electron pair with it to furnish a bond to the neighboring or adjacent carbocation. The shifted alkyl group and the positive charge of the carbocation switch positions on the moleculeReactions of tertiary carbocations react much faster than that of secondary carbocations. We see alkyl shift from a secondary carbocation to tertiary carbocation in S N 1 reactions: We observe slight variations and differences between the two reactions.
After protonating the alcohol substrate to form the alkyloxonium ion, the water must leave at the same time as the alkyl group shifts from the adjacent carbon to skip the formation of the unstable primary carbocation. Carbocation Rearrangements for E1 Reactions E1 reactions are also affected by alkyl shift. Once again, we can see both minor and major products. However, we see that the more substituted carbons undergo the effects of E1 reactions and furnish a double bond. See practice problem #4 below for an example as the properties and effects of carbocation rearrangements in E1 reactions are similar to that of alkyl shifts.
1,3-Hydride and Greater Shifts Typically, hydride shifts can occur at low temperatures. However, by heating the solutionf of a cation, it can easily and readily speed the process of rearrangement. One way to account for a slight barrier is to propose a 1,3-hydride shift interchanging the functionality of two different kinds of methyls. Another possibility is 1,2 hydride shift in which you could yield a secondary carbocation intermediate. Then, a further 1,2 hydride shift would give the more stable rearranged tertiary cation. More distant hydride shifts have been observed, such as 1,4 and 1,5 hydride shifts, but these arrangements are too fast to undergo secondary cation intermediates.
Analogy Carbocation rearrangements happen very readily and often occur in many organic chemistry reactions. Yet, we typically neglect this step. Dr. Sarah Lievens, a Chemistry professor at the University of California, Davis once said carbocation rearrangements can
This is partly because in more --substituted alkenes, the p orbitals of the pi bond are stabilized by neighboring alkyl substituents, a phenomenon similar to hyperconjugation. Hydride and Alkyl Shifts
Since the dehydration reaction of alcohol has a carbocation intermediate, hydride or alkyl shifts can occur which relocates the carbocation to a more stable position. The dehydrated products therefore are a mixture of alkenes, with and without carbocation rearrangement. Tertiary cation is more stable than secondary cation, which in turn is more stable than primary cation due to a phenomenon known as hyperconjugation, where the interaction between the filled orbitals of neighboring carbons and the singly occupied p orbital in the carbocation stabilizes the positive charge in carbocation. - In hydride shifts, a secondary or tertiary hydrogen from a carbon next to the original carbocation takes both of its electrons to the cation site, swapping place with the carbocation and renders it a more stable secondary or tertiary cation.
The lone pair of electrons on oxygen atom makes the –OH group weakly basic. Oxygen can donate two electrons to an electron-deficient proton. Thus, in the presence of a strong acid, R—OH acts as a base and protonates into the very acidic alkyloxonium ion + OH 2 (The pKa value of a tertiary protonated alcohol can go as low as -3.8). This basic characteristic of alcohol is essential for its dehydration reaction with an acid to form alkenes. Mechanism for the Dehydration of Alcohol into Alkene Different types of alcohols may dehydrate through a slightly different mechanism pathway.
However, the general idea behind each dehydration reaction is that the –OH group in the alcohol donates two electrons to H + from the acid reagent, forming an alkyloxonium ion. This ion acts as a very good leaving group which leaves to form a carbocation. The deprotonated acid (the nucleophile) then attacks the hydrogen adjacent to the carbocation and form a double bond. Primary alcohols undergo bimolecular elimination ( E2 mechanism ) while secondary and tertiary alcohols undergo unimolecular elimination ( E1 mechanism ).
The relative reactivity of alcohols in dehydration reaction is ranked as the following Methanol < primary < secondary < tertiary Primary alcohol dehydrates through the E2 mechanism Oxygen donates two electrons to a proton from sulfuric acid H 2 SO 4 , forming an alkyloxonium ion. Then the nucleophile HSO 4 – back-side attacks one adjacent hydrogen and the alkyloxonium ion leaves in a concerted process, making a double bond. Secondary and tertiary alcohols dehydrate through the E1 mechanism Similarly to the reaction above, secondary and tertiary –OH protonate to form alkyloxonium ions.
However, in this case the ion leaves first and forms a carbocation as the reaction intermediate. The water molecule (which is a stronger base than the HSO 4 - ion) then abstracts a proton from an adjacent carbon, forming a double bond. Notice in the mechanism below that the aleke formed depends on which proton is abstracted: the red arrows show formation of the more substituted 2-butene, while the blue arrows show formation of the less substituted 1-butene. Recall the general rule that more substituted alkenes are more stable than less substituted alkenes, and trans alkenes are more stable than cis alkenes.
Trans-substituted alkenes reduce this effect of spatial interference by separating the two bulky substituents on each side of the double bond (for further explanation on the rigidity of C=C bond, see Structure and Bonding in Ethene- The pi Bond). Heats of hydrogenation of differently-substituted alkene isomers are lowest for more-substituted alkenes, suggesting that they are more stable than less-substituted alkenes and thus are the major products in an elimination reaction. This is partly because in more --substituted alkenes, the p orbitals of the pi bond are stabilized by neighboring alkyl substituents, a phenomenon similar to hyperconjugation .
Hydride and Alkyl Shifts Since the dehydration reaction of alcohol has a carbocation intermediate, hydride or alkyl shifts can occur which relocates the carbocation to a more stable position. The dehydrated products therefore are a mixture of alkenes, with and without carbocation rearrangement . Tertiary cation is more stable than secondary cation, which in turn is more stable than primary cation due to a phenomenon known as hyperconjugation, where the interaction between the filled orbitals of neighboring carbons and the singly occupied p orbital in the carbocation stabilizes the positive charge in carbocation.
In hydride shifts , a secondary or tertiary hydrogen from a carbon next to the original carbocation takes both of its electrons to the cation site, swapping place with the carbocation and renders it a more stable secondary or tertiary cation. Similarly, when there is no hydride available for hydride shifting, an alkyl group can take its bonding electrons and swap place with an adjacent cation, a process known as alkyl shift. Practice Problems Test your understanding by predicting what product(s) will be formed in each of the following reactions: 1 . 2. Solutions 1. Did you notice the reaction temperature?
It is only 25°, which is much lower than the required temperature of 170°C for dehydration of primary alcohol. This reaction will not produce any alkene but will form ether. 2. . Notice that the reactant is a secondary -OH group, which will form a relatively unstable secondary carbocation in the intermediate. Thus hydride shift from an adjacent hydrogen will occur to make the carbocation tertiary, which is much more stable. The products are a mixture of alkenes that are formed with or without carbocation rearrangement (A number of products are formed faster than hydride shift can occur). References Vollhart, K. Peter C. and Neil Schore. Organic Chemistry, Structure and Function.
primary, secondary, and tertiary compounds and carbocations are shown in the accompanying … example of a tertiary (3°) chloride is (CH3)3CC1, t- butylchloride. When this tertiary chloride … or tertiary — would be least likely to give a molecular ion? Solution Because tertiary carbocations
The kinetics for surface-catalyzed alcohol dehydration reactions often depend on the structure of the alcohol. Studies of structure–activity relations across primary, secondary, and tertiary alcohols can provide fundamental information on the nature of active sites on the surface. Here in this paper, we investigated the dehydration of 1-butanol, 2-butanol, and tert-butanol over TiO 2 anatase catalysts modified with various phosphonic acid (PA) self-assembled monolayers (SAMs). As a response to the presence of PAs, the three C4 alcohol isomers showed different dehydration rates, with 1-butanol dehydration being enhanced to the greatest extent by PA modification. Furthermore, the fluorinated, more polar 4-fluorobenzylphosphonic acid outperformed alkyl PAs across all alcohols. Steady-state kinetic measurements and temperature-programmed desorption studies indicated that PA SAMs significantly lowered the dehydration activation barrier; the extent of reduction in the barrier was sensitive to both the substitution of the alcohol and the charge distribution on the PA in a way that was consistent with stabilization of a carbenium-like transition state. Overall, the effect of PA modifiers on alcohol dehydration rates was found to be determined from a balance between transition state stabilization and active site blocking effects, with the potential to tune activity and selectivity based on the structure and coverage of the SAM.
AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Everything we examined (7) — 5 independent sources
This check searched the claim as stated. It did not run a separate search for evidence against it.