- Properties of alkyl halides
- Boiling and melting points
- Polarity
- Solvent power
- Nomenclature
- Example
- Obtaining
- Halogenation with light or ultraviolet radiation
- Adding hydrocids or halogens to alkenes
- Reactions
- Nucleophilic substitution
- Examples
- Elimination
- Synthesis of Grignard reagents
- Examples
- Applications
- Solvent
- Organic synthesis
- Pharmaceutical industry
- Refrigerants
- References
The alkyl halides are organic compounds in which a carbon atom sp hybridized 3 is covalently bound to a halogen (F, Cl, Br, I). From another perspective, it is convenient, to simplify, to assume that they are haloalkanes; These are alkanes to which some H atoms have been replaced by halogen atoms.
Also, as its name indicates, the halogen atoms must be linked to alkyl groups, R, to be considered of this type of halide; although, structurally they can be substituted or branched and have aromatic rings, and still remain an alkyl halide.
1-Chlorobutane molecule, an example of an alkyl halide. Source: Gabriel Bolívar.
Above is the 1-chlorobutane molecule, which corresponds to one of the simplest alkyl halides. It can be seen that all its carbons have single bonds, and also have sp 3 hybridization. Therefore, the green sphere, corresponding to the Cl atom, is linked to a skeleton derived from alkane butane.
Even simpler examples than 1-chlorobutane are those derived from methane gas: the smallest hydrocarbon of all.
From its CH 4 molecule, H atoms can be replaced by, that is, iodine. If you substitute an H, you will have CH 3 I (iodomethane or methyl iodide). By substituting two H, you will have CH 2 I 2 (diiodomethane or methylene iodide). Then and finally, the Is replace all the Hs giving CHI 3 (iodoform), and CI 4 (carbon tetraiodide).
Alkyl halides are characterized by being very reactive and, by having the most electronegative atoms in the periodic table, it is suspected that through endless mechanisms they exert an influence on biological matrices.
Properties of alkyl halides
The properties of this family of compounds depend on their molecular structures. However, when compared with those of its derived alkanes, notable differences can be observed caused by the simple fact of having CX bonds (X = halogen atom).
That is, the CX bonds are responsible for any difference or similarity between one or more alkyl halides.
To begin with, CH bonds are almost apolar, given the small electronegativity difference between C and H; In contrast, CX bonds have a permanent dipole moment, because halogens are more electronegative than carbon (especially fluorine).
On the other hand, some halogens are light (F and Cl), while others are heavy (Br and I). Their atomic masses also shape differences within the CX bonds; and in turn, directly on the properties of the halide.
Therefore, adding halogens to a hydrocarbon is equal to increasing its polarity and molecular mass; it is equal to making it less volatile (up to a point), less flammable, and increasing its boiling or melting points.
Boiling and melting points
Having said the above, the size and therefore the weight of the different halogens are shown in increasing order:
F <Cl <Br <I
Thus, alkyl halides containing F atoms can be expected to be lighter than those containing Br or I atoms.
For example, some halides derived from methane are considered:
CH 3 F <CH 3 Cl <CH 3 Br <CH 3 I
CH 2 F 2 <CH 2 Cl 2 <CH 2 Br 2 <CH 2 I 2
And so on for the other derivatives product of a higher degree of halogenation. Note that the order is maintained: fluorine halides are lighter than iodine halides. Not only that, but also their boiling and melting points also obey this order; RF boils at lower temperatures than RI (R = CH 3, for this case).
Likewise, all those liquids are colorless, since electrons in their CX bonds cannot absorb or release photons to transit other energy levels. However, as they get heavier they can crystallize and display colors (as does iodoform, CHI 3).
Polarity
CX bonds differ in polarity, but in the reverse order as above:
CF> C-Cl> C-Br> CI
Therefore, CF bonds are more polar than CI bonds. Being more polar, RF halides tend to interact through dipole-dipole forces. Meanwhile, in the RBr or RI halides, their dipole moments are weaker and the interactions governed by the London dispersion forces gain greater strength.
Solvent power
As the alkyl halides are more polar than the alkanes from which they are derived, they increase their ability to dissolve a greater number of organic compounds. It is for this reason that they tend to be better solvents; although, it does not mean that they can supplant alkanes in all applications.
There are technical, economic, ecological and performance criteria to prefer a halogenated solvent over an alkane.
Nomenclature
There are two ways to name an alkyl halide: by its common name, or by its systematic name (IUPAC). Common names are usually more convenient to use when RX is simple:
CHCl 3
Chloroform: common name
Methyl trichloride or trichloromethane: IUPAC name.
But systematic names are preferable (and the only option), when you have branched structures. Ironically, common names come in handy again when structures are too complicated (like the ones you'll see in the last section).
The rules for naming a compound according to the IUPAC system are the same as for alcohols: the main chain is identified, which is the longest or the most branched. The carbons are then listed starting from the end closest to the substituents or branches, named in alphabetical order.
Example
To illustrate, we have the following example:
Alkyl halide as an example of nomenclature. Source: Gabriel Bolívar.
The first branch is the methyl group at C-4; but, as there is a double bond, it gains higher priority over the stated rule. For this reason, the longest chain begins to be listed from the right, headed by a carbon atom linked to two halogens: Cl and Br.
With the enumeration, the substituents are named in alphabetical order:
1-bromo-1-chloro-4-methyl-2-hexene.
Obtaining
To obtain the alkyl halides, the molecules must be subjected to a halogenation process; that is, incorporating halogen atoms into their structures, especially an atom with a carbon sp 3.
There are two methods to obtain or synthesize them: by means of ultraviolet radiation through a mechanism by free radicals, or by means of the addition of hydro acids or halogens.
Halogenation with light or ultraviolet radiation
The first, the least appropriate and with the worst performance, consists in irradiating the alkanes with ultraviolet radiation (hv) in the presence of a halogen. For example, the equations for the chlorination of methane are shown:
CH 4 + Cl 2 => CH 3 Cl + HCl (under ultraviolet light)
CH 3 Cl + Cl 2 => CH 2 Cl 2 + HCl
CH 2 Cl 2 + Cl 2 => CHCl 3 + HCl
CHCl 3 + Cl 2 => CCl 4 + HCl
The four compounds (CH 3 Cl, CH 2 Cl 2, CHCl 3 and CCl 4) are formed, and therefore there is a mixture, which can be subjected to fractional distillation. However, this method is impractical, and recourse to organic syntheses is preferred.
Another example is the bromination of n-hexane:
CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 + Br 2 => CH 3 (Br) CHCH 2 CH 2 CH 2 CH 3 + HBr
Again, in this reaction, light or ultraviolet radiation is used to promote the formation of free radicals. Bromine, as it is a deep red liquid, becomes discolored as it reacts, thus observing a color change from red to colorless when 2-bromohexane is formed.
Adding hydrocids or halogens to alkenes
The second method of obtaining alkyl halides consists of treating alcohols (ROH) or alkenes (R 2 C = CR 2) with hydracids. Hydracids have the general formula HX (HF, HCl, HBr and HI). An example will be shown using ethanol for each of them:
CH 3 CH 2 OH + HF => CH 3 CH 2 F + H 2 O
CH 3 CH 2 OH + HCl => CH 3 CH 2 Cl + H 2 O
CH 3 CH 2 OH + HBr => CH 3 CH 2 Br + H 2 O
CH 3 CH 2 OH + HI => CH 3 CH 2 I + H 2 O
Likewise, alkenes can add HX molecules to their double bonds, forming secondary alkyl halides.
CH 2 = CH-CH 3 + HBr => BrCH 2 -CH 2 -CH 3 + CH 3 -CHBr-CH 3
The product BrCH 2 -CH 2 -CH 3 is 1-bromopropane, and CH 3 -CHBr-CH 3 is 2-bromopropane. The second is the majority product because it is the one with the greatest stability, while the first is produced to a lesser extent because it is more unstable. This is because CH 3 CHBrCH 3 is a secondary alkyl halide.
Very similar occurs when what is added to the alkene is a molecule of X 2:
CH 2 = CH-CH 3 + Br 2 => BrCH 2 -CHBr-CH 3
However, an alkyl halide is obtained with two bromine atoms bonded to adjacent carbons; vicinal alkyl halide. If, on the other hand, you had the two bromines attached to the same carbon, you would have a geminal alkyl halide, like the following:
Br 2 CH-CH 2 -CH 3
Reactions
Nucleophilic substitution
The reactivities of alkyl halides are based on the brittleness or strength of the CX bond. The heavier the halogen, the weaker the bond, and therefore the more easily it will break. In a chemical reaction bonds are broken and new ones are formed; the CX bonds are broken, to form a CG bond (G = new group).
In more appropriate terms, X acts as a leaving group, and G as an entering group in a nucleophilic substitution reaction. Why does this reaction occur? Because X, being more electronegative than carbon, "steals" electron density, leaving it with a deficit of electrons that translates as a positive partial charge:
C δ + -X δ-
If a negative (: G -) or neutral species with a pair of electrons available (: G), capable of forming a more stable CG bond, hovers around the vicinity, X will end up being replaced by G. The above can be represented by the following equation chemistry:
RX +: G - => RG + X -
The weaker the CX or RX bond, the greater its reactivity or tendency to be replaced by the nucleophilic (or nucleophile) agent G; that is, lovers of nuclei or positive charges.
Examples
Below is a series of general equations for the nucleophilic substitutions that alkyl halides can undergo:
RX + OH - => ROH + X - (Alcohols)
+ OR '- => ROR ' (Ethers, Williamson synthesis)
+ I - => RI (Alkyl iodides)
+ CN - => RCN (Nitriles)
+ R'COO - => RCOOR '(Esters)
+ NH 3 => RNH 2 (Amines)
+ P (C 6 H 5) 3 => RP (C 6 H 5) 3 + X - (Phosphonium salts)
+ SH - => RSH (Thiols)
From these examples one can already suspect how valuable alkyl halides are for organic syntheses. One of the many substitutions that remained to be cited is that of the Friedel Crafts reaction, used to "rent" aromatic rings:
RX + ArH + AlCl 3 => ArR
In this reaction, an H of the aromatic ring is replaced by an R group from RX.
Elimination
Alkyl halides can release HX molecules through an elimination reaction; specifically, a dehydrohalogenation:
R 2 CH-CXR 2 + OH - => R 2 C = CR 2 + HX
Dehydrohalogenation is said to occur because both H and X are lost in the same HX molecule.
Synthesis of Grignard reagents
Alkyl halides can react with certain metals to form the Grignard reagent, used to add R groups to other molecules. The general equation for its synthesis is as follows:
RX + Mg => RMgX
Examples
Various examples of alkyl halides have already been mentioned throughout the sections. Some others, simple, are:
-Ethyl chloride, CH 3 CH 2 Cl
-Isopropyl fluoride, (CH 3) 2 CH 2 F
-2-methyl-3-chloropentane, CH 3 -CH (CH 3) -CHCl-CH 2 CH 3
-secbutyl iodide, CH 3 CH 2 CH 2 I-CH 3
-3-bromo-6-iodoheptane, CH 3 -CH 2 -CHBr-CH 2 -CH 2 -CH 2 I
-3,4-dibromo-1-pentene, CH 3 -CHBr-CHBr-CH = CH 2
Applications
Solvent
In previous sections, mention was made of the solvent capacity of the alkyl halides. The industry has taken advantage of this property to use them as cleaners, whether for textile materials, electronic components, or to remove varnish stains.
Likewise, they are used as solvents for paints, or for organic or greasy samples for innumerable types of analytical tests.
Organic synthesis
Alkyl halides are very useful for "alkylating" aromatic rings, while serving as a starting source for the synthesis of practically all other families of organic compounds. Synthetically, RX is considered a source of R groups or chains, which may be desired for incorporation into highly aromatic compounds.
Pharmaceutical industry
It was mentioned at the beginning that halogen atoms interact with biological matrices, so that in our organisms they cannot go unnoticed without generating a change, positive or negative. If a drug exerts a positive effect on the body, having a halogen atom this effect may or may not be increased.
Then, if X is directly linked to a carbon with sp 3 hybridization, you will have an alkyl halide and not a halogenated derivative. Some of such halides are shown below in the following series of images:
Phenoxybenzamine, a medicine used to fight blood pressure in patients with pheochromocytoma. Source: Utent: Mark Pea.
Isoflurane, an inhalation anesthetic. Source: Benjah-bmm27.
Clindamycin, an antibiotic. Source: M mitcheltree.
Pimecrolimus, used to treat atopic dermatitis. Can you locate the chlorine atom? Source: MarinaVladivostok.
Halomon, possible antitumor agent and alkyl halide from seaweed Portieria hornemannii, a natural source. Source: Jü
Note that in these five drugs there is at least one CH 2 -X or CH-X bond; that is, the halogen is attached to an sp 3 carbon.
Refrigerants
The famous refrigerant Freon-12 (CHCIF 2), like other fluoroalkanes or hydrofluorocarbons, replaced ammonia gases and chlorofluorocarbons (CFCs) in this function because, even though they are non-volatile and non-toxic substances, they destroy the ozone layer; while Freon-12, being more reactive, is destroyed before reaching such altitudes.
References
- Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
- Clark Jim. (2016, July 14). Uses of Alkyl Halides. Chemistry LibreTexts. Recovered from: chem.libretexts.org
- Gál, B., Bucher, C., & Burns, NZ (2016). Chiral Alkyl Halides: Underexplored Motifs in Medicine. Marine drugs, 14 (11), 206. doi: 10.3390 / md14110206
- Alkyl Halides. Recovered from: chemed.chem.purdue.edu
- Patkar Prachi. (July 16, 2017). All About Alkyl Halides: Properties, Uses, and Much More. Science Struck. Recovered from: sciencestruck.com
- R. Ship. (2016). Alkyl Halides. Recovered from: hyperphysics.phy-astr.gsu.edu
- Learning Guide for Chapter 9 - Alkyl Halides I. Recovered from: cactus.dixie.edu
- QA Eduardo Vega Barrios. (sf). Alkyl halides: Properties, uses and applications. [PDF. Recovered from: cvonline.uaeh.edu.mx