Describe the main differences between Sn1 and Sn2 substitution reactions, and describe the trends for transition-state stability/reactivity.
Sn1 substitution occurs via a stable intermediate, a carbocation, in a two-step pathway. Generally, the reactant gets deprotonated in the first step to form a carbocation in which the positive charge is retained on this intermediate substrate. The second step involves a nucleophile donating electron charge to form the product. The rate determining step is unimolecular. Reactivity: allylic ~ tertiary > secondary > primary substrates. Sn2 substitution occurs via a one-step pathway, in which the nucleophile bonds to the reactant and the reactant leaving group leaves in the same step (concerted mechanism). Due to this kinetic behavior, Sn2 reaction rate is bimolecular and does not proceed through a stable intermediate. Reactivity: allylic ~ primary > secondary > tertiary substrates.
When elements combine to form compounds, ionic or covalent bonding can occur. Define ionic and covalent compounds, and describe characteristics of each bonding type.
Ionic compounds are formed when electrons are donated from one atom to another in order to form a bond. Examples include metals forming salts (Na Cl). Characteristics of ionic compounds include: reacting of positive and negative ions to form ionic bonds; formation of solid salts; high melting and boiling points; strong attraction force between particles to form lattice structures; ionic compounds separate (dissociate) in water to form electrolyte solutions that can conduct electricity. Covalent compounds are formed when electrons are shared between atoms to form a bond. Examples include non-metals forming neutral compounds (H2O, water). Characteristics of covalent compounds include: reacting of non-metal atoms to form neutral compounds as solids, liquids, or gases; low melting and boiling points; relatively weak intermolecular attractive forces; do not form electrolyte solutions in water.
Define the principle of "matter" and how we classify the different states of matter.
"Matter" refers to anything that takes up space and contains mass. All matter can be classified into the states of solid, liquid, or gas based on pre-determined physical properties that come from the way atoms are arranged or "packed" together. Tightly packed atomic arrangement leads to solids (retains shape), less-tightly packed atomic arrangement yields liquids (changes shape), and very loosely packed atomic arrangements give gases (no shape).