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Using mm-wavelength data from ALMA, the EMoCA spectral line survey revealedthe presence of both the straight-chain and branched forms of propyl cyanide(C$_3$H$_7$CN) toward the Galactic Center star-forming source Sgr B2(N2). Thiswas the first interstellar detection of a branched aliphatic molecule. Throughcomputational methods, we seek to explain the observed $i$:$n$ ratio for propylcyanide, and to predict the abundances of the four different forms of thehomologous nitrile, butyl cyanide (C$_4$H$_9$CN). We also investigate whetherother molecules will show a similar degree of branching, by modeling alkanes upto pentane (C$_5$H$_{12}$). We use a coupled three-phase chemical kineticsmodel to simulate the chemistry of Sgr B2(N2), using an updated chemicalnetwork that includes grain-surface/ice-mantle formation routes for branchednitriles and alkanes. We use the EMoCA survey data to search for thestraight-chain form of butyl cyanide toward Sgr B2(N2). The observed $i$:$n$ratio for propyl cyanide is reproduced by the models. Butyl cyanide ispredicted to show similar abundances to propyl cyanide, and to exhibit strongbranching, with the $sec$ form clearly dominant over all others. The additionof CN to acetylene and ethene is found to be important to the production ofvinyl, ethyl, propyl, and butyl cyanide. We report a non-detection of$n$-C$_4$H$_9$CN toward Sgr B2(N2), with an abundance at least 1.7 times lowerthan that of $n$-C$_3$H$_7$CN. This value is within the range predicted by thechemical models. The models indicate that the degree of branching rises withincreasing molecular size. The efficiency of CN addition to unsaturatedhydrocarbons boosts the abundances of nitriles in the model, and enhances theratio of straight-to-branched molecule production. The predicted abundance of$s$-C$_4$H$_9$CN makes it a good candidate for future detection toward SgrB2(N2).

Exploring molecular complexity with ALMA (EMoCA): Simulations of branched carbon-chain chemistry in Sgr B2(N)