The alkyl halides are organic compounds in which a carbon atom with sp hybridization3 is covalently linked 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it is convenient, to simplify, to assume that they are haloalkanes; these are alkanes to which some H atoms have been replaced by halogen atoms.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, 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.
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 hybridization3. 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 molecule of CH4 H atoms can be replaced by, say, iodine. If an H is substituted, we will have the CH3I (iodomethane or methyl iodide). By substituting two H, we will have the CHtwoItwo (diiodomethane or methylene iodide). Then and finally, the I's replace all the H's by giving CHI3 (iodoform), and CI4 (carbon tetraiodide).
Alkyl halides are characterized by being very reactive and, by having the most electronegative atoms in the periodic table, it is intuited that through endless mechanisms they exert an influence on biological matrices.
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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 C-X bonds (X = halogen atom).
That is, the C-X bonds are responsible for any difference or similarity between one or more alkyl halides..
To begin with, the C-H bonds are almost apolar, given the small electronegativity difference between C and H; on the other hand, the C-X bonds present a permanent dipole moment, due to the fact that 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 C-X 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; equates to making it less volatile (up to a point), less flammable, and increasing its boiling or 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:
CH3F < CH3Cl < CH3Br < CH3I
CHtwoFtwo < CHtwoCltwo < CHtwoBrtwo < CHtwoItwo
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 = CH3, for this case).
Likewise, all those liquids are colorless, since electrons cannot absorb or release photons in their C-X bonds to transit other energy levels. However, as they get heavier they can crystallize and display colors (as does iodoform, CHI3).
C-X bonds differ in polarity, but in the reverse order as above:
C-F> C-Cl> C-Br> C-I
Therefore, C-F bonds are more polar than C-I bonds. Being more polar, RF halides tend to interact through dipole-dipole forces. Meanwhile, in RBr or RI halides, their dipole moments are weaker and the interactions governed by the London dispersion forces gain greater strength..
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.
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:
CHCl3
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 are useful again when the structures are too complicated (like the ones that will be seen 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. Then, the carbons are listed starting from the end closest to the substituents or branches, named in alphabetical order.
To illustrate it, we have the following example:
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.
To obtain the alkyl halides, the molecules must be subjected to a halogenation process; that is, incorporate halogen atoms into their structures, especially an atom with a carbon sp3.
There are two methods to obtain or synthesize them: by ultraviolet radiation through a free radical mechanism, or by adding hydrocids or halogens..
The first, the least appropriate and with the worst performance, consists of irradiating the alkanes with ultraviolet radiation (hv) in the presence of a halogen. For example, the equations for the chlorination of methane are shown:
CH4 + Cltwo => CH3Cl + HCl (under UV light)
CH3Cl + Cltwo => CHtwoCltwo + HCl
CHtwoCltwo + Cltwo => CHCl3 + HCl
CHCl3 + Cltwo => CCl4 + HCl
The four compounds (CH3Cl, CHtwoCltwo, CHCl3 and CCl4) are formed, and therefore you have 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:
CH3CHtwoCHtwoCHtwoCHtwoCH3 + Brtwo => CH3(Br) CHCHtwoCHtwoCHtwoCH3 + 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.
The second method of obtaining alkyl halides consists of treating alcohols (ROH) or alkenes (RtwoC = CRtwo) with hydracids. Hydracids have the general formula HX (HF, HCl, HBr and HI). An example will be shown using ethanol for each of them:
CH3CHtwoOH + HF => CH3CHtwoF + HtwoOR
CH3CHtwoOH + HCl => CH3CHtwoCl + HtwoOR
CH3CHtwoOH + HBr => CH3CHtwoBr + HtwoOR
CH3CHtwoOH + HI => CH3CHtwoI + HtwoOR
Likewise, alkenes can add HX molecules to their double bonds, forming secondary alkyl halides..
CHtwo= CH-CH3 + HBr => BrCHtwo-CHtwo-CH3 + CH3-CHBr-CH3
The BrCH producttwo-CHtwo-CH3 is 1-bromopropane, and CH3-CHBr-CH3 2-bromopropane. The second is the majority product because it is the most stable, while the first is produced to a lesser extent because it is more unstable. This is because the CH3CHBrCH3 is a secondary alkyl halide.
Very similar occurs when what is added to the alkene is a molecule of Xtwo:
CHtwo= CH-CH3 + Brtwo => BrCHtwo-CHBr-CH3
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:
BrtwoCH-CHtwo-CH3
The reactivities of alkyl halides are based on the brittleness or strength of the C-X 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; C-X bonds are broken, to form a C-G bond (G = new group).
In more appropriate terms, X acts as a leaving group, and G as an entering group in a reaction of nucleophilic substitution. Why does this reaction occur? Because X, being more electronegative than carbon, "steals" electron density, leaving it a deficit of electrons that translates as a positive partial charge:
Cδ+-Xδ-
If a negative species (: G-) or neutral with an available electron pair (: G), capable of forming a more stable C-G bond, X will end up being replaced by G. The above can be represented by the following chemical equation:
RX +: G- => RG + X-
The weaker the C-X or R-X bond, the greater its reactivity or tendency to be replaced by the nucleophilic agent (or nucleophile) G; that is, lovers of nuclei or positive charges.
A series of general equations for the nucleophilic substitutions that alkyl halides can undergo is shown below:
RX + OH- => ROH + X- (Alcohols)
+ OR'- => ROR' (Ethers, Williamson synthesis)
+ I- => RI (Alkyl iodides)
+ CN- => RCN (Nitriles)
+ R'COO- => RCOOR '(Esters)
+ NH3 => RNHtwo (Amines)
+ P (C6H5)3 => RP (C6H5)3+ X- (Phosphonium salts)
+ SH- => RSH (Thiols)
From these examples it can already be suspected 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 + AlCl3 => ArR
In this reaction, an H of the aromatic ring is replaced by an R group from the RX.
Alkyl halides can release HX molecules through an elimination reaction; specifically, a dehydrohalogenation:
RtwoCH-CXRtwo + Oh- => RtwoC = CRtwo + HX
It is said that dehydrohalogenation occurs because both an H and an X are lost in the same HX molecule.
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
Various examples of alkyl halides have already been mentioned throughout the sections. Some others, simple, are:
-Ethyl chloride, CH3CHtwoCl
-Isopropyl fluoride, (CH3)twoCHtwoF
-2-methyl-3-chloropentane, CH3-CH (CH3) -CHCl-CHtwoCH3
-sec-butyl iodide, CH3CHtwoCHtwoI-CH3
-3-bromo-6-iodoheptane, CH3-CHtwo-CHBr-CHtwo-CHtwo-CHtwoI
-3,4-dibromo-1-pentene, CH3-CHBr-CHBr-CH = CHtwo
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, either for textile materials, electronic components, or to remove varnish stains..
Likewise, they are used as solvents for paints, or organic or greasy samples for innumerable types of analytical tests..
Alkyl halides are very useful for "alkylating" aromatic rings, while serving as a starting source for the synthesis of virtually 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..
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.
So if X is directly bonded to a sp-hybridized carbon3, it will have an alkyl halide and not a halogenated derivative. Some of such halides are shown below in the following series of images:
Note that in these five drugs there is at least one CH-type bondtwo-X or CH-X; that is, the halogen is attached to an sp carbon3.
The famous refrigerant Freon-12 (CHCIFtwo), like other fluoroalkanes or hydrofluorocarbons, replaced ammonia gases and chlorofluorocarbons (CFCs) in this function because, although they are non-volatile or toxic substances, they destroy the ozone layer; while freon-12, being more reactive, is destroyed before reaching such altitudes.
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