Name: nucleophilic strength in substitution reactions
Uploaded: 2023-04-30T18:28:35.000Z
Duration: 2 min 30 s
Channel: JoVE

In una reazione di sostituzione nucleofila, la molecola reagente che sposta il gruppo uscente nel substrato è chiamata nucleofilo.
I nucleofili sono specie ricche di elettroni e, per definizione, basi di Lewis. L'atomo nucleofilo ha un'alta densità di elettroni e dona i suoi elettroni a un centro parzialmente positivo, carente di elettroni, creando così un nuovo legame.
I reagenti nucleofili sono anionici o neutri. I nucleofili anionici sono ioni negativi contenenti una o più coppie solitarie di uguale energia, di solito su eteroatomi. Le coppie solitarie che non si legano occupano orbitali molecolari ad alta energia, il che le rende meno stabili e più reattive.
Ad esempio, il nucleofilo anionico del carbonio, come lo ione cianuro, ha una coppia solitaria sia sul carbonio che sull'azoto. Tuttavia, poiché l'orbitale sp del carbonio ha un'energia più alta di quella dell'azoto, il carbonio è il centro nucleofilo.
I nucleofili neutri hanno una o più coppie di elettroni non condivisi sugli orbitali molecolari più alti occupati, per lo più, di eteroatomi.
Inoltre, in specie come gli alcheni che non hanno coppie solitarie, una regione ad alta densità elettronica - il legame pi greco - funziona come sito nucleofilo.
Un nucleofilo neutro è meno nucleofilo della sua forma anionica, a causa dell'assenza di una carica negativa.
In generale, i valori di pKa degli acidi possono essere utilizzati per valutare la forza delle loro basi coniugate o nucleofili. Nel caso di molecole contenenti lo stesso atomo nucleofilo, più alto è il pKas dei loro acidi coniugati, più forte sarà il nucleofilo.     
Il prodotto di una reazione di sostituzione nucleofila dipende dal tipo di nucleofilo utilizzato.
Quando un nucleofilo anionico reagisce con il substrato, il legame covalente formato neutralizza la carica formale del nucleofilo, risultando in un prodotto neutro.
In confronto, quando un nucleofilo neutro reagisce con il substrato, il nucleofilo acquisisce una carica formale positiva. Una fase di deprotonazione che segue completa la reazione, risultando in un prodotto neutro.

nucleofili

Nucleophiles

La parola “nucleofilo” ha una radice greca e si traduce in amante del nucleo. I nucleofili sono specie cariche negativamente o neutre con una coppia di elettroni in un orbitale molecolare occupato ad alta energia (HOMO). Poiché queste specie tendono a donare coppie di elettroni, anche i nucleofili sono considerati basi di Lewis. Le specie cariche negativamente, come OH−, Cl− o HS−, con una o più coppie di elettroni, sono tipicamente nucleofili. Allo stesso modo, specie neutre come ammoniaca, ammine, acqua e alcol hanno coppie solitarie di elettroni che non si legano e possono agire come nucleofili. Inoltre, le molecole senza una coppia solitaria di elettroni possono ancora agire come nucleofili, come gli alcheni e gli anelli aromatici con orbitali π di legame.

La forza relativa di un nucleofilo di spostare un gruppo uscente in una reazione di sostituzione è chiamata nucleofilicità. Le specie caricate negativamente sono più nucleofile delle loro controparti neutre. Come regola empirica, maggiore è il pKa di un acido coniugato, migliore è il nucleofilo. Ad esempio, lo ione idrossido, una base coniugata dell'acqua (pKa 15,7), è un nucleofilo migliore dello ione acetato, una base coniugata dell'acido acetico (pKa~5).

Poiché la nucleofilicità non è una proprietà intrinseca data ad una specie specifica, è influenzata da molti fattori, incluso il tipo di solvente in cui viene condotta la reazione. Nei solventi protici polari, l’elevata solvatazione degli anioni riduce la disponibilità del nucleofilo a partecipare alle reazioni di sostituzione.

Quando si confrontano gli alogenuri, il fluoruro, essendo l'anione più piccolo e più elettronegativo, è il più forte solvatato, mentre lo ioduro, lo ione più grande e meno elettronegativo, è il meno solvatato. Pertanto, nei solventi protici polari, lo ioduro è il miglior nucleofilo. Nei solventi polari aprotici, invece, gli anioni sono “nudi” a causa della scarsa solvatazione e possono partecipare liberamente ad un attacco nucleofilo. Nei solventi polari aprotici, la basicità del nucleofilo determina la sua nucleofilicità rendendo il fluoruro il miglior nucleofilo.

Inoltre, la polarizzabilità degli atomi influisce sulla nucleofilicità. La polarizzabilità descrive la facilità con cui gli elettroni nella nuvola possono essere distorti. Un nucleofilo con un atomo grande ha una maggiore polarizzabilità, il che significa che può donare una densità elettronica maggiore all'elettrofilo rispetto adun atomo piccolo, i cui elettroni sono trattenuti più strettamente.

Nucleofili

La parola "nucleofilo" ha una radice greca e si traduce in amante del nucleo. I nucleofili sono specie caricate negativamente o neutre con un ...

Education

Didattica

JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

Reazioni di sostituzione nucleofila ed eliminazione degli alogenuri alchilici

nucleophilic-substitution-and-elimination-reactions-of-alkyl-halides

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

nucleophilic strength in substitution reactions

Nucleophiles are electron-pair donors that take part in substitution reactions. The word comes from Greek and means nucleus-loving. Because they donate electrons, nucleophiles are also Lewis bases.

Many nucleophiles are negatively charged species, such as hydroxide, chloride, and hydrosulfide. Neutral molecules can also act as nucleophiles when they have lone pairs of electrons. Examples include ammonia, amines, water, and alcohols.

Some molecules without a lone pair can still behave as nucleophiles. Alkenes and aromatic rings can donate electrons from their bonding pi orbitals. The strength of a nucleophile is called nucleophilicity, and it describes how well the species can displace a leaving group in a substitution reaction.

In general, negatively charged species are more nucleophilic than similar neutral species. A useful rule is that a higher pKa of the conjugate acid often means a better nucleophile. For example, hydroxide, the conjugate base of water with a pKa of 15.7, is a better nucleophile than acetate, the conjugate base of acetic acid with a pKa of about 5.

Nucleophilicity also depends on the solvent. In polar protic solvents, anions are strongly solvated, which lowers their ability to react. Among halides in these solvents, fluoride is solvated the most and iodide the least, so iodide is the better nucleophile. In polar aprotic solvents, anions are less solvated and more free to attack, so basicity becomes the main guide and fluoride is the best nucleophile.

Atom size also matters because of polarizability. Polarizability is how easily an electron cloud can be distorted. Larger atoms are more polarizable, so they can donate electron density to an electrophile more easily than smaller atoms with more tightly held electrons.

Nucleophilic Strength in Substitution Reactions

nucleophilic-strength-in-substitution-reactions

alkyl halides

Alkyl Halides: Structure and Reactivity

Alkyl halides are halogen-substituted alkanes. In these compounds, one or more hydrogen atoms in an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon bonded to the halogen is sp3-hybridized and has a tetrahedral shape. Not every carbon-halogen compound is an alkyl halide. If the halogen is attached to an sp2-hybridized carbon in a carbon-carbon double bond, the compound is a vinyl halide. If the halogen is attached to an sp2-hybridized carbon in an...

nucleophilic substitution reactions

SN1 and SN2 in Organic Substitution

Nucleophilic substitution reactions replace one group in a molecule with another. In these reactions, a nucleophile, which is a Lewis base, reacts with an electrophile, which is a Lewis acid. The nucleophile takes the place of a halogen atom bonded to carbon and releases a stable ion called the leaving group. This reaction pattern is closely related to Lewis acid-base chemistry. The electron-rich species acts as the nucleophile. The electron-deficient species acts as the electrophile. A key...

nucleophiles

Nucleophiles are electron-pair donors that take part in substitution reactions. The word comes from Greek and means nucleus-loving. Because they donate electrons, nucleophiles are also Lewis bases. Many nucleophiles are negatively charged species, such as hydroxide, chloride, and hydrosulfide. Neutral molecules can also act as nucleophiles when they have lone pairs of electrons. Examples include ammonia, amines, water, and alcohols. Some molecules without a lone pair can still behave as...

electrophiles

Electrophile Reactivity in Organic Reactions

Electrophile reactivity in organic reactions depends on how a species attracts electrons. Electrophiles are electron-poor and are key partners in nucleophilic substitution reactions. Their charge and orbital structure help explain why they react the way they do. Electrophiles can be positive or neutral species. A positive electrophile such as a proton reacts because it has a vacant, low-energy 1s orbital. Other positive electrophiles, such as carbocations, are reactive because they have an...

leaving groups

How Leaving Groups Affect Substitution

Leaving groups affect how nucleophilic substitution reactions happen. In this type of reaction, a nucleophile replaces a functional group on a substrate. The group that leaves is called the leaving group, and it departs when the bond breaks by heterolytic cleavage, taking both electrons with it. A good leaving group is usually a stable weak base. It can leave as an anion or as a neutral molecule. The easier it is for the leaving group to leave, the more it can influence the reaction rate. <br...

carbocations

Carbocation Structure and Stability

Carbocations are reactive carbon species formed during some nucleophilic substitution and elimination reactions. They are electron-deficient intermediates, with the central carbon holding only six electrons and three bonded atoms. Because they are short-lived and highly reactive, they play an important role in many organic reaction pathways. The central carbon in a carbocation is sp2 hybridized and has a trigonal planar shape. It also contains an empty p orbital that sits perpendicular to the...

sn2 reaction: kinetics

SN2 Rate Law and Reaction Mechanism

SN2 reaction kinetics links reaction rate to the concentration of reactants. In a chemical reaction, kinetic studies measure how fast reactants disappear or products appear under fixed conditions such as temperature, pressure, and solvent. These studies help chemists find the rate law, which can give clues about the species involved in the rate-determining step and the reaction mechanism. The rate of a reaction is found experimentally. Scientists withdraw small samples, called aliquots, at...

sn2 reaction: mechanism

SN2 Backside Attack and Inversion

SN2 reactions happen in one step and end with inversion of configuration. The nucleophile and the substrate both take part in the rate-determining step, and no intermediate forms. The substrate contains a polarized carbon-halide bond. The more electronegative halogen pulls the electron cloud toward itself. That leaves the carbon atom electrophilic, or electron-poor, with a partial positive charge. The halide has a partial negative charge. The electrophilic carbon attracts the nucleophile,...

sn2 reaction: transition state

SN2 Reaction: Reactivity and Steric Effects

An SN2 reaction of an alkyl halide happens in one step. The nucleophile forms a bond with the electrophilic carbon while the halide leaves at the same time. No intermediate forms during this process. As the nucleophile approaches with its lone pairs, the leaving group moves away with the electron pair that was bonded to carbon. The transition state is shown with dotted partial bonds and square brackets. This state is very unstable, so it quickly changes into the more favored product state. <br...

sn2 reaction: stereochemistry

SN2 Inversion of Configuration

SN2 reactions cause inversion of configuration at the carbon being substituted. The nucleophile attacks from the back side, while the leaving group departs at the same time. This happens through a transition state, where the carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral. The change in shape helps explain the product stereochemistry. If the substrate is achiral at the alpha-carbon, the inversion is not seen. When the alpha-carbon is chiral, the reversal is...

sn1 reaction: kinetics

SN1 Reaction Rate and Unimolecular Step

SN1 reaction rate depends on the substrate, not the nucleophile. This pattern was identified by Sir Christopher Ingold and Edward D. Hughes while they studied nucleophilic substitution reactions of tertiary alkyl halides. A tertiary alkyl halide can still undergo substitution even when the nucleophile is weak. In their kinetic studies, the rate of product formation stayed the same when the nucleophile concentration changed, as long as the substrate concentration remained constant. That means...

sn1 reaction: mechanism

SN1 Reaction Steps and Carbocation Formation

The SN1 reaction follows a step-by-step pathway that begins with carbocation formation. In a polar protic solvent, a tertiary haloalkane can ionize so that only the substrate takes part in the rate-determining step, which is the slow step. First, the haloalkane undergoes heterolytic cleavage to form a carbocation intermediate and a halide ion. This bond break is highly endothermic and has a large activation energy. The polar protic solvent helps stabilize the ions by solvation, and that makes...

sn1 reaction: stereochemistry

SN1 Pathway and Product Chirality

SN1 reactions can change the chirality of a product because the reaction goes through a carbocation intermediate. First, the bond between the electrophilic carbon and the leaving group ionizes. Then the nucleophile attacks the carbocation. The carbocation formed in an SN1 reaction is sp2 hybridized and has a trigonal planar shape. Its three substituents lie in the same plane, which creates a plane of symmetry. That makes the carbocation achiral, so the nucleophile can approach from either side...

predicting products: sn1 vs. sn2

SN1 and SN2 Reaction Factors

Nucleophilic substitution of alkyl halides can follow an SN1 or SN2 pathway. In an SN2 reaction, the nucleophile attacks as the leaving group leaves at the same time. In an SN1 reaction, the substrate first splits to form a carbocation, which is a positively charged intermediate. The structure of the alkyl halide helps decide which mechanism is more likely. As alkyl substitution increases, steric hindrance also increases, which makes backside attack harder for SN2. More substituted alkyl...

elimination reactions

E2 vs E1 in Alkyl Halide Elimination

Elimination reactions of alkyl halides form alkenes by removing groups from adjacent carbons. A base can act on the substrate instead of causing substitution. It deprotonates the neighboring carbon, and the reaction creates at least one pi bond. The carbon bonded to the halogen is the alpha carbon, and the adjacent carbon is the beta carbon. These are also called beta elimination or 1,2-elimination reactions. Strong bases help drive this process. Common examples include hydroxides, alkoxides,...

e2 reaction: kinetics and mechanism

E2 Elimination: Rate, Base, and Solvent

E2 elimination is a concerted reaction of alkyl halides that forms an alkene. In this mechanism, the base removes a beta hydrogen as the halide leaves from the alpha carbon. The two bonds change at the same time, so the products are formed in one step. The rate-limiting transition state has partially broken carbon-hydrogen and carbon-halogen bonds. It also has a partially formed pi bond between the alpha and beta carbons. Because the beta hydrogen and the halide are eliminated together, the...

e2 reaction: stereochemistry and regiochemistry

e2-reaction-stereochemistry-and-regiochemistry

E2 Elimination: Major and Minor Products

E2 elimination reactions of alkyl halides can form one or more alkenes, and the product mix depends on regiochemistry and stereochemistry. Regiochemistry controls where the double bond appears. Stereochemistry controls the shape of the alkene that forms. When a substrate has two different beta hydrogens, a strong base can remove either one. That can give two regioisomeric alkenes. The more substituted alkene is the major product and is called the Zaitsev product. The less substituted alkene is...

e1 reaction: kinetics and mechanism

E1 Elimination: Carbocation Stability and Rates

E1 elimination depends on carbocation stability and reaction rate. It follows a two-step path. The first step is the rate-limiting loss of the leaving group. The second step is the abstraction of the beta hydrogen by a weak base. Experimental support for the E1 mechanism comes from kinetic studies and isotope studies. Kinetic studies show first-order kinetics, which means the rate depends only on substrate concentration. Isotope studies show that hydrogen is removed only in the second step. <br...

e1 reaction: stereochemistry and regiochemistry

e1-reaction-stereochemistry-and-regiochemistry

E1 Elimination: Zaitsev Product and Carbocation

E1 elimination often gives the most substituted alkene, called the Zaitsev product. In the example, water acts as a weak base and favors elimination over substitution, so two alkene products can form. The Zaitsev alkene is usually more stable than the Hofmann product because it has more alkyl groups attached to the double bond. The lower-energy pathway also helps explain why the Zaitsev product is favored. Its transition state has lower energy, so this product is both thermodynamically stable...

predicting products: substitution vs. elimination

predicting-products-substitution-vs-elimination

Choosing Products in Substitution and Elimination

Substitution and beta-elimination compete when a nucleophile reacts with an alkyl halide. The major product depends on the substrate, the nucleophile, the temperature, and the solvent. Because more than one pathway can happen at once, a reaction often gives a product mixture. The structure of the alkyl halide helps predict which mechanism is more likely. Methyl halides and 1° halides usually favor bimolecular reactions, especially SN2. SN1 and E1 are disfavored for 1° halides because the 1°...

key terms and concepts

Learn how nucleophiles donate electron pairs, why charge and pKa matter, and how solvent and polarizability change nucleophilic strength.

Nucleophilic Strength in Substitution Reactions | JoVE K12

organic chemistry

JoVE Core Organic Chemistry spiega i concetti di base della stereochimica, delle reazioni e dei meccanismi usando video-lezioni concise e facili da capire. Inoltre, i video scientist-in-action dimostrano l'applicazione di tali concetti in esperimenti di ricerca classici e originali eseguiti al giorno d'oggi nei laboratori in tutto il mondo.

Nucleophilic Strength in Substitution Reactions

Description

Transcript

Tags