4-Benzyloxyaniline hydrochloride
[CAS 51388-20-6]

Identification

• Classification: Organic raw materials >> Amino compound >> Acyclic monoamines, polyamines and their derivatives and salts
• Name: 4-Benzyloxyaniline hydrochloride
• Synonyms: 4-(Benzyloxy)aniline hydrochloride; 4-Aminophenyl benzyl ether hydrochloride
• Molecular Formula: C₁₃H₁₃NO.HCl
• Molecular Weight: 235.71
• CAS Registry Number: 51388-20-6
• EC Number: 257-170-6
• SMILES: C1=CC=C(C=C1)COC2=CC=C(C=C2)N.Cl

Properties

• Melting point: 228 °C (Decomposes) (Expl.)

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H302-H315-H317-H319-H335
• Safety Statements: P261-P264-P264+P265-P270-P271-P272-P280-P301+P317-P302+P352-P304+P340-P305+P351+P338-P319-P321-P330-P332+P317-P333+P317-P337+P317-P362+P364-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Eye irritation	Eye Irrit.	2	H319
Skin irritation	Skin Irrit.	2	H315
Specific target organ toxicity - single exposure	STOT SE	3	H335
Skin sensitization	Skin Sens.	1	H317
Acute toxicity	Acute Tox.	4	H302
Eye irritation	Eye Irrit.	2A	H319

chemBlink Chemical Story

4-Benzyloxyaniline hydrochloride, CAS 51388-20-6, is an aromatic amine salt used as an intermediate in organic and pharmaceutical synthesis. Its molecular formula is C₁₃H₁₄ClNO and its molecular weight is 235.71. The compound can also be represented as C₁₃H₁₃NO·HCl, emphasizing that it is the hydrochloride salt of 4-benzyloxyaniline.

The structure contains two benzene rings connected through a -CH₂-O- linkage. One ring also carries an amino group in the para position relative to the benzyloxy substituent. This arrangement makes the molecule useful in synthesis because the amino group can participate in further transformations while the oxygen remains protected as a benzyl ether.

The word "benzyloxy" is especially important here. A benzyl group, C₆H₅CH₂-, is commonly used to protect alcohols and phenols. Converting an O-H group into an O-CH₂C₆H₅ group prevents the oxygen from participating in many reactions that would otherwise affect the free hydroxyl group.

This is one of the central ideas of protecting-group chemistry. A functional group may be needed in the final molecule but inconvenient during an earlier reaction. Instead of permanently removing it, chemists temporarily change its identity, carry out the required chemistry elsewhere, and later restore the original group.

A published process for 4-benzyloxyaniline hydrochloride demonstrates this strategy very clearly. The synthesis begins with 4-nitrophenol. At this stage, the molecule already contains the oxygen that will eventually become a phenolic hydroxyl group and the nitrogen that will ultimately become an amine, although the nitrogen is initially present as a nitro group.

In the first step, 4-nitrophenol is reacted with benzyl bromide in the presence of potassium carbonate and tetrabutylammonium bromide. The phenolic oxygen is benzylated, producing 4-benzyloxynitrobenzene.

This transformation changes O-H into O-CH₂C₆H₅. The oxygen remains in exactly the same position on the aromatic ring, but its chemical behavior changes because it is now present as an ether rather than a free phenol.

The nitro group is then reduced to an amino group. In the published process, tin(II) chloride is used in an ethanol-water system containing hydrochloric acid. The aromatic nitro group is converted into NH₂ while the benzyl ether remains intact.

This selectivity is the reason protecting groups are so useful. The chemist wants one part of the molecule to change while another part survives. Here, the nitrogen functionality undergoes a major transformation, from NO₂ to NH₂, without removing the benzyl group from oxygen.

The free 4-benzyloxyaniline is then treated with dilute hydrochloric acid at 20-30 °C to form 4-benzyloxyaniline hydrochloride. This last operation does not rebuild the carbon skeleton. It is an acid-base reaction in which the basic aniline nitrogen is protonated and the chloride ion becomes the counterion.

Why make the hydrochloride salt instead of simply isolating the free amine?

Organic amines are frequently handled as salts because salt formation can change practical physical properties such as crystallinity, melting behavior, solubility, and ease of isolation. Whether a particular salt is superior in every respect depends on the compound and application, but converting an amine into a crystalline hydrochloride is a common strategy in laboratory and industrial chemistry.

The published process provides an unusually detailed view of how this relatively simple intermediate can be manufactured. In one example, benzylation of 4-nitrophenol produced the protected nitro compound in 91.58% yield. Reduction then gave 4-benzyloxyaniline in 83.08% yield, and conversion to the hydrochloride gave the salt in 91.11% yield.

A second example reported 93.40%, 84.11%, and 92.19% for the corresponding three operations. The patent describes the overall process as giving a total molar yield above 68%.

The same patent identifies 4-benzyloxyaniline hydrochloride as an important pharmaceutical and organic-synthesis intermediate and specifically describes it as a raw material for preparing 4-benzyloxyphenylhydrazine hydrochloride.

That transformation introduces another useful piece of synthetic chemistry. Aromatic amines can be converted through diazonium chemistry into other nitrogen-containing functional groups, including hydrazine derivatives. The para-benzyloxy substituent can remain present while the nitrogen functionality is further manipulated.

The benzyl ether can also serve another purpose later in a synthetic sequence. Benzyl protecting groups are valued because they can often be removed by hydrogenolysis, typically using hydrogen in the presence of a suitable metal catalyst. Cleavage of the O-benzyl bond regenerates the phenolic O-H group.

This gives the molecular design a useful logic. The phenolic oxygen can be protected as a benzyl ether while chemistry is performed on the nitrogen-containing end of the molecule. When the synthetic sequence eventually requires the free phenol, the benzyl group can be removed under appropriate conditions.

It is important, however, not to assume that every reaction sequence containing 4-benzyloxyaniline hydrochloride necessarily ends with debenzylation. The compound is a general synthetic intermediate, and the fate of its benzyl group depends on the target molecule and reaction route.

The hydrochloride form adds another layer of temporary chemical control. The benzyl group controls the reactivity of oxygen through covalent protection, while protonation controls the state of the amino group through reversible acid-base chemistry.

Thus, one small molecule illustrates two very different ways chemists manage functional groups. One involves making and later breaking a covalent bond. The other simply changes protonation state by adding or removing acid.

4-Benzyloxyaniline hydrochloride therefore provides a useful snapshot of molecular planning. The benzene framework is already in place, the oxygen is temporarily protected, the nitrogen has been converted into an amine, and the amine has been isolated in its hydrochloride form.

In multistep synthesis, success often depends not only on creating new bonds but also on preventing the wrong bonds from forming at the wrong time. Protecting groups and salt formation may appear to be supporting operations, but they are often what allow the main synthetic sequence to proceed cleanly and reproducibly.

References

1. PubChem. 4-Benzyloxyaniline hydrochloride, CID 2723831. CAS 51388-20-6. Molecular formula C₁₃H₁₄ClNO; molecular weight 235.71.

2. CN102001955A. Method for synthesizing 4-benzyloxy aniline hydrochloride. Preparation from 4-nitrophenol through benzylation, reduction, and hydrochloride formation.

3. Thermo Scientific Chemicals. 4-Benzyloxyaniline hydrochloride, CAS 51388-20-6. Chemical identity and specifications.

4. Wuts, P. G. M. Greene's Protective Groups in Organic Synthesis. Chemistry of benzyl protecting groups for alcohols and phenols.

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