Name: alkyl halides: structure and reactivity
Uploaded: 2023-04-30T18:28:35.000Z
Duration: 2 min 45 s
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Alkyl halides, or haloalkanes, are organic compounds comprised of an alkyl group&#160;and a halogen atom bonded to an sp3 hybridized carbon.
Each carbon atom is defined by its proximity to the halogen atom. The carbon bonded directly to the halogen is denoted &#8220;alpha-carbon&#8221;; the carbon next to it is called &#8220;beta-carbon&#8221; and so forth.
Depending on the number of alkyl groups on the alpha carbon, alkyl halides are further classified as primary, secondary, and tertiary.
Nomenclature of alkyl halides follows similar IUPAC rules as for alkanes. Halogens are regarded as substituents and named fluoro-, chloro-, bromo-, and iodo-.
First, count the carbons on the longest chain. Here, five corresponds to pentane. Secondly, find and name the substituents. Here it is bromo-, chloro-, and methyl.
Next, number the parent chain and assign a locant to the substituents giving them the lowest possible number. Lastly, assemble the substituents alphabetically. This compound is called 4-bromo-1-chloro-4-methyl pentane.
If the chain contains a double bond, the double bond has precedence and obtains the lowest number giving the compound 3-fluoro-1-butene.
If a molecule is cyclic, chiral, and with multiple substituents, the substituents are likewise assigned the lowest locant possible, and the groups are alphabetically ordered. The stereocenters are indicated at the beginning of the name, giving (1R,2R)-3-butyl-1,2-dichloro-5-ethylcyclohexane.
Alkyl halides are used as building blocks in syntheses because of their reactivity. Going down group 17, the halogen&#8217;s atom size and bond length increase, while electronegativity and basicity decrease.
The higher electronegativity of halogens leads to the withdrawal of electron density from the adjacent carbon making the alpha carbon electrophilic and, hence, more reactive.
The alkyl halide&#8217;s reactivity is further influenced by the stability of the halogen&#8217;s conjugate base. Iodide, bromide, and chloride are stable yet weak conjugate bases of their strong hydrohalic acids. Because of their stability, they can be more easily substituted by a stronger base, making them good leaving groups.

alkylhalogeniden

Alkyl Halides

Structurele eigenschappen

Alkylhalogeniden zijn met halogeen gesubstitueerde alkanen waarin één of meer waterstofatomen van een alkaan zijn vervangen door een halogeenatoom zoals fluor, chloor, broom of jodium. Het koolstofatoom in een alkylhalogenide is gebonden aan het halogeenatoom, dat sp^3-gehybridiseerd is en een tetraëdrische vorm vertoont.

In tegenstelling tot alkylhalogeniden worden verbindingen waarin een halogeenatoom gebonden is aan een sp^2 -gehybridiseerd koolstofatoom van een dubbele koolstof-koolstof binding (C=C) vinylhalogeniden genoemd. Terwijl arylhalogeniden verbindingen zijn waarin een halogeenatoom gebonden is aan een sp^2 -gehybridiseerd koolstofatoom van een aromatische ring.

Bovendien worden alkylhalogeniden, afhankelijk van de substitutiegraad aan het koolstofatoom dat het halogeen bevat, geclassificeerd in primaire, secundaire en tertiaire alkylhalogeniden. Het koolstofatoom gebonden aan een halogeenatoom wordt het α-koolstofatoom genoemd. Het koolstofatoom dat aan de α-koolstof is gekoppeld wordt β-koolstof genoemd. De aangrenzende koolstofatomen worden vervolgens aangegeven met letters van het Griekse alfabet (γ, δ, enzovoort).

Nomenclatuur

De naamgeving van alkylhalogeniden volgt de algemene IUPAC-regels van de organische chemie:

Identificeer de langste koolstofketen en geef deze een naam.
Zoek en benoem de substituent(en).
Nummer de langste koolstofketen en wijs een nummer, of locant, aan voor elke substituent.
Verzamel de substituenten in alfabetische volgorde.

Reactiviteit en toepassingen

Alkylhalogeniden zijn veelzijdige verbindingen die dienen als oplosmiddelen, pesticiden en tussenproducten bij de bereiding van kleurstoffen, medicijnen en synthetische polymeren. De chemische reactiviteit van alkylhalogeniden verschilt afhankelijk van de structurele classificatie. Alkylhalogeniden worden geclassificeerd in primaire, secundaire en tertiaire alkylhalogeniden, afhankelijk van de mate van substitutie op een koolstofhoudend halogeen.

Bovendien speelt de elektronegativiteit van het halogeenatoom een belangrijke rol in de reactiviteit van het alkylhalogenide. Behalve jodium hebben andere halogenen een aanzienlijk grotere elektronegativiteit dan koolstof. Als gevolg van verschillen in elektronegativiteit tussen koolstof- en halogeenatomen is de covalente binding tussen de atomen gepolariseerd; dus de koolstof draagt een deels positieve lading en het halogeenatoom een deels negatieve lading. Bijgevolg is de koolstof die aan het halogeen is gebonden een elektrofiel.

In een periodiek systeem, dalend in de halogeenfamilie, neemt de elektronegativiteit af; omgekeerd neemt de grootte van het halogeenatoom toe. De lengte van de koolstof-halogeenbinding neemt dus toe, terwijl de sterkte en polariteit van de binding afnemen van fluor naar jodium, waardoor het gemakkelijker wordt om de binding te verbreken.

Afgezien van deze factoren beïnvloedt de relatieve stabiliteit van de overeenkomstige geconjugeerde basen, of halogenide-anionen, ook de chemische aard van het alkylhalogenide. De stabiliteit van halogenide-anionen kan worden gemeten met betrekking tot de relatieve zuurgraad van de waterstofhalogeniden. Het sterkste waterstofhalogenidezuur, waterstofjodide, heeft een pK_a van -11. Dit betekent dat het volledig dissocieert met de meest stabiele geconjugeerde base, het jodide-ion en een proton. Jodium is dus een uitstekende vertrekkende groep. Behalve fluorwaterstofzuur met een pK_a van 3,2 hebben andere halogeenwaterstofzuren een pK_a-waarde van minder dan 0, waardoor ze sterke zuren zijn met zwakke en gestabiliseerde geconjugeerde basen, die uitstekende vertrekkende groepen zijn.

Alkylhalogeniden

Structural Properties
Alkyl halides are halogen-substituted alkanes wherein one or more hydrogen atoms of an alkane is replaced by a halogen atom such as ...

Education

JoVE Core

Organic Chemistry

Organische chemie

Unit 1

Onderdeel 1

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

Nucleofiele substitutie- en eliminatiereacties van alkylhaliden

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

KEY TERMS AND CONCEPTS

BELANGRIJKE TERMEN EN CONCEPTEN

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 aromatic ring, the compound is an aryl halide. Alkyl halides are also grouped as primary, secondary, or tertiary based on how substituted the carbon bearing the halogen is.

The carbon attached to the halogen is called the alpha-carbon. The carbon next to it is the beta-carbon. The nearby carbons are then labeled with Greek letters such as gamma and delta. Alkyl halides are named using standard IUPAC rules. First, identify the longest carbon chain and name it. Then name the substituents, assign locants, and list the substituents in alphabetical order.

Alkyl halides are useful as solvents, pesticides, and starting materials for dyes, drugs, and synthetic polymers. Their reactivity depends on their structure and on the halogen atom. Because halogens are usually more electronegative than carbon, the carbon-halogen bond is polarized. This gives the carbon a partial positive charge, so it acts as an electrophile.

Halogen trends also matter. Moving down the halogen family, electronegativity decreases and atomic size increases. As a result, the carbon-halogen bond gets longer, weaker, and less polar from fluorine to iodine, so it is easier to break. The stability of the matching halide anion also affects reactivity. Hydroiodic acid is the strongest hydrohalic acid, with a pKa of -11, so iodide is a very stable conjugate base and an excellent leaving group. With the exception of hydrofluoric acid, which has a pKa of 3.2, the other hydrohalic acids have pKa values below 0 and form weak, stabilized conjugate bases that leave easily.

Alkyl Halides: Structure and Reactivity

alkyl halides

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

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...

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 alkyl halides are built, named, and classified, and why bond polarity and leaving-group strength affect their reactivity in organic chemistry.

Alkyl Halides: Structure and Reactivity | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Alkyl Halides: Structure and Reactivity

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