  Interstellar carbonaceous dust is mainly formed in the innermost regions of
circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars.
In these highly chemically stratified regions, atomic and diatomic carbon,
along with acetylene are the most abundant species after H$_2$ and CO. In a
previous study, we addressed the chemistry of carbon (C and C$_2$) with H$_2$
showing that acetylene and aliphatic species form efficiently in the dust
formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene
is known to be a key ingredient in the formation of linear polyacetylenic
chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by
previous experiments. However, these experiments have not considered the
chemistry of carbon (C and C$_2$) with C$_2$H$_2$.
  In this work, by employing a sufficient amount of acetylene, we investigate
its gas-phase interaction with atomic and diatomic carbon. We show that the
chemistry involved produces linear polyacetylenic chains, benzene and other
PAHs, which are observed with high abundances in the early evolutionary phase
of planetary nebulae. More importantly, we have found a non-negligible amount
of pure and hydrogenated carbon clusters as well as aromatics with aliphatic
substitutions, both being a direct consequence of the addition of atomic
carbon. The incorporation of alkyl substituents into aromatics can be
rationalized by a mechanism involving hydrogen abstraction followed by methyl
addition. All the species detected in gas phase are incorporated into the
nanometric sized dust analogues, which consist of a complex mixture of sp,
sp$^2$ and sp$^3$ hydrocarbons with amorphous morphology.
