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This is the nitrosonium ion, which is produced by the reaction of $\ce{NaNO2}$ and $\ce{HCl}$:

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[Image source: Wikipedia]

And this is the reaction between aniline and nitrosonium:

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[Image source: Wikipedia]

Cannot it take place as an EAS, and with substitution at the ortho or para positions? The amino group is a very good activating group for EAS after all.

Update: I forgot to mention that EAS is observed in case $\ce{-NR2}$ is attached to the ring in place of $\ce{-NH2}$.

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    $\begingroup$ You certainly could. If I had to offer a post hoc justification, it's probably that at 0 degrees, the rate of ring substitution is extremely slow and cannot compete with N-attack. There is a similar selectivity question in, for example, the acylation of phenol. chemistry.stackexchange.com/questions/31531/… $\endgroup$ – orthocresol Jul 15 '16 at 9:26
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What actually happens is that the nitrosonium ion always adds faster to the amino nitrogen. But that reaction is rapidly reversible unless a proton can dissociate from the amino nitrogen to give a stable N-nitroso product.

A primary or secondary amine has the required N-H proton, and in the case of a primary amine the presence of a second N-H proton enables further reaction making a diazonium ion (as shown for aniline by the OP) or even molecuar nitrogen. But a tertiary amine has no N-H proton for the elimination step. A tertiary amine then undergoes no net nitrosation reaction at the nitrogen atom, but if it's aromatic then EAS emerges as an alternative.

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