Use greek in newcommand, LaTeX

Question

I have problems with inserting the characters of the Greek alphabet in the source code. I like to use Greek also in mathematical formulas, but as you can see from the photo when you finish the compilation, the Greek changes to the normal Latin alphabet. Besides this problem, I don't understand why in the \newcommand it doesn't accept Greek in order to rename old commands.

For example, why can't I use:

\newcommand{∂}{\partial}??? (in the case I can do it, how should I?)

code:

\documentclass{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage{sidenotes}
\usepackage[italian, greek]{babel}
\newcommand{\fk}{\displaystyle\frac}
\newcommand{\MU}{\MakeUppercase} 
\newcommand{\mc}{\mathcal}
\newcommand{\mb}{\mathbb}
\newcommand{\Σ}{\Sigma}
\begin{document}
\tableofcontents
\chapter{}
[
d σκ  \Σ
]
ξηκξαντονελλο μεψψαρειλλο
\end{document}