Unit 14: Applications of Organic Chemistry
Learning Aim A: Understand the structures, reactions and properties of functional group compounds.
Assignment title: Functional group chemistry for designer molecules
Halogenoalkanes: Alkanes in which one or more hydrogen atoms
are replaced by halogen atoms produce haloalkanes, which are
organic molecules. Alkyl halides and haloalkanes are other names
for haloalkanes. Simply put, they are alkanes in which one or more
hydrogen atoms have been replaced by halogen atoms. Group 7 of
the periodic table contains halogens. Group 17 is another name for
this unit. All halogens exhibit strong electronegativity because they
each have seven electrons in their outer shell. Haloalkanes with a
single halogen atom have the typical molecular formula CnH2n+1X.
Chloroethane (C2H5Cl) and bromomethane (CH3Br) are two https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/bromomethane_1.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-
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examples. There are three different types of haloalkanes. Primary,
secondary, tertiary. This depends on the number of alkyl groups
attached to the carbon's C-X bond. Classifications are indicated by
the degree symbol (°) as shown above. Alkyl groups are often
referred to as R groups in chemical molecules. The carbon attached
to the halogen atom of the primary haloalkane (1°) is also attached
to either zero or the R group. Halogen-bonded carbons of
secondary haloalkanes (2°) are also attached to two R groups.
Three R groups are also attached to the carbon atom attached to
the halogen atom of the tertiary haloalkane (3°). Below is a list of
examples of primary, secondary, and tertiary haloalkanes. The R https://studysmarter-mediafiles.s3.amazonaws.com/media/1865576/summary_images/
halogenoalkane_classification.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-
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identification of the halogen atoms in the molecule.
Due to the polar C-X bond, haloalkanes exhibit very different properties from alkanes. The sole reason
for this is the difference in electronegativity between halogen and carbon. All halogens are more
electronegative than carbon. As a result, the halogen atoms are partially negatively charged and the
carbon atoms are partially positively charged. Partial charges are indicated by a delta symbol () above
each atom. Due to the polarity of haloalkanes, a constant voltage is applied between the dipoles. These
affect some physical properties of the molecule. They are stronger than molecules, so they require more
effort to resist van der Waals forces. This property is common to haloalkanes and their parent alkanes. It
is almost tasteless, colorless and hydrophobic. The melting and boiling points of chloro, bromo, and
iodoalkanes are higher than similar alkanes, depending on their atomic weight and amount of halide.
This phenomenon is caused by stronger intermolecular forces (London dispersion and dipole
interactions). Unlike carbon tetrachloride (CCl4), carbon tetraiodide (CI4) is a solid. There are almost
always exceptions to such broad concepts. Some fluoroalkanes have lower melting and boiling points
than their non-halogenated counterparts. The reason for this is the decrease in the polarizability of
fluorine. For example, methane (CH4) melts at a temperature of -182.5 °C and tetrafluoromethane melts
,at -183.6 °C. Reactivity towards the starting alkane is also increased by the presence of halogens (other
than fluorine).
Nucleophilic substitution in halogenoalkanes: In nucleophilic substitution, the nucleophile donates two
electrons to the partially positively charged carbon of the haloalkane. The chalcogen bond is
heterolytically broken when the halide ion accepts both electrons. A carbon atom and a nucleophile form
a new bond. Another group replaces the halogen. To complete these nucleophilic substitution reactions,
the bond between carbon and halogen must be broken. The harder it is to break, the slower it reacts.
The carbon-fluorine bond is stronger and less brittle than the C–H bond. The carbon-fluorine bond has
the highest polarity, but the strength of the bond has a large impact on the degree of reactivity.
Therefore, fluoroalkanes are very inert. For various haloalkanes, the compound becomes weaker as you
go from chlorine to bromine to iodine. As can be observed, iodoalkanes react even faster than
bromoalkanes, while chloroalkanes react the slowest.
Nucleophilic Substitution: Primary Halogenoalkanes: Leading haloalkanes and nucleophiles react in a
single step because the C-halo bond is severed when the C-nucleophilic bond forms. Species called
nucleophiles (ions or molecules) are drawn to positively charged areas of other substances by strong
attraction. Either totally anionic nucleophiles or those with a significant intramolecular charge. Water,
ammonia, cyanide ions, and oxygen ions are typical nucleophiles. This mechanism is also known as
bimolecular nucleophilic substitution (SN2) because the amount of both reactants (haloalkane and
nucleophile) determines the reaction rate. This results in a transient, inseparable high-energy transition
state that decays quickly.
A typical large haloalkane is presented as an example, such as
bromoethane. In bromoethane, carbon and bromine are joined by
a polar bond. Examine its response to the typical nucleophilic ion
Nu-. It has at least one lone pair of electrons. For example, Nu-
could be OH- or CN-. The lone pair of Nu ions moves towards the +
carbon due to its strong attractive force and begins to form
coordinative (coordination) bonds. As a result, as the electrons of
the C-Br bond approach bromine, bromine becomes more and
more negative. This reaction continues until bromine is completely
liberated as a Br ion and tightly bound to carbon. From the other https://www.chemguide.co.uk/mechanisms/nucsub/
side of the bromine atom, the nu ion approaches the + carbon. This is sn2geneq2.gif
because the large bromine atom can block incoming nuclear radiation and repel attacks from the
flanks. It is obvious that the C-Br bond is halfway between partial bond breaking and partial bonding for
carbon and nuclei. The migratory period is when this happens. It cannot be isolated, not even
temporarily. Simply said, it's the neutral ground between skillfully assaulting one group and fleeing from
another.
Nucleophilic substitution: Secondary halogenoalkanes: Secondary haloalkanes are employed using
both techniques. When reacting, certain molecules use the SN2 mechanism whereas others use the SN1
mechanism. The molecule's numerous alkyl groups allow an approaching nucleophile to continue
accessing the + carbon atom, perhaps resulting in an SN2 reaction. The SN1 procedure is practical
because the secondary carbocations generated in the slow phase are more stable than the original
, carbocations. The SN1 route, however, is less reliable than the tertiary pathway and, as a result, is less
effective than tertiary haloalkanes.
Nucleophilic substitution: Tertiary halogenoalkanes: Three alkyl groups and a
halogen are joined to a carbon atom in tertiary haloalkanes. These alkyl groups
might be similar or dissimilar. But in this piece, we'll concentrate on the
straightforward instance of (CH3)3CBr, also known as 2-bromo-2-methylpropane.
Nucleophiles that are approaching the + carbon atom from the opposite side of
the halogen atom attack primary haloalkanes. In contrast, tertiary haloalkanes
do not experience this. The CH3 groups cover the whole backside of the molecule. https://www.chemguide.co.uk/
Other methods are hindered by the bromine atom, thus a different mechanism is mechanisms/nucsub/clutter.GIF
required.
The response takes place in two stages. Some of the haloalkanes are ionised in the first process,
resulting in carbocations and bromide ions.
Tertiary carbocations are more stable than
secondary or primary carbocations, making this reaction conceivable. But as soon as the carbocation is
created, it starts to react when it comes into touch with nucleophiles like Nu-. A new bond is created
when the positive carbon advances in that direction and encounters the nucleophile's lone pair.
The pace at which the haloalkane ionises
controls how quickly the process moves forward. SN1 (substitution, nucleophilic, one species
participating in the first slow phase) is the name of the procedure in question. This is so because he only
includes one species at this point.
Nucleophilic Substitution: Halogenoalkane with aqueous NaOH: Refluxing a haloalkane in a solution of
sodium or potassium hydroxide converts the halogen to -OH, forming an alcohol. A condenser should be
placed vertically in the flask to prevent volatiles from escaping from the mixture during reflux. Since they
are all soluble, common solvents are ethanol and water in a 50/50 ratio.
Water cannot dissolve haloalkanes. When using only water as the solvent, the haloalkane and sodium
hydroxide solution do not mix, so the reaction only occurs when the two layers are in contact. As an
example, consider the basic haloalkane 1-bromopropane.
