Penicillin G (C₁₆H₁₈N₂O₄S) — Properties, Uses & Safety

Overview

Penicillin G (C16H18N2O4S), also known as benzylpenicillin, is the first true antibiotic ever discovered and one of the most important drugs in medical history. With a molar mass of 334.39 g/mol, it is a white crystalline powder that revolutionized medicine when Alexander Fleming discovered it in 1928 after observing that the mold Penicillium notatum killed bacteria on a culture plate. Howard Florey and Ernst Boris Chain later developed it into a usable drug, and all three shared the 1945 Nobel Prize in Physiology or Medicine. The key structural feature of penicillin is its beta-lactam ring — a strained four-membered ring containing a nitrogen atom. This ring is essential for penicillin's mechanism of action: it irreversibly binds to and inhibits transpeptidase enzymes (also called penicillin-binding proteins) that bacteria need to cross-link peptidoglycan chains in their cell walls. Without functional cell walls, bacteria cannot maintain their structural integrity and die. This mechanism makes penicillin effective against many gram-positive bacteria, including Streptococcus, Staphylococcus, and Clostridium species. Penicillin G is administered by injection because it is acid-labile and would be destroyed by stomach acid if taken orally. It is used to treat strep throat, syphilis, meningitis, endocarditis, and other serious bacterial infections. The development of antibiotic resistance is a major concern — some bacteria produce beta-lactamase enzymes that break open penicillin's beta-lactam ring, rendering it ineffective. This has led to the development of modified penicillins (like amoxicillin and methicillin) and beta-lactamase inhibitors. Safety considerations include allergic reactions, which affect about 10% of the population and can range from mild rashes to life-threatening anaphylaxis. GHS classifications include health hazard and irritant warnings. For chemistry students, penicillin connects organic chemistry concepts including ring strain in the beta-lactam ring, amide bond chemistry, chirality (penicillin has multiple stereocenters), and the relationship between molecular structure and biological function. The story of penicillin's discovery and development is also a compelling narrative about the role of serendipity and perseverance in scientific breakthroughs.

IUPAC Name

(2S,5R,6R)-3,3-dimethyl-7-oxo-6-(2-phenylacetamido)-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid

CAS Number

61-33-6

PubChem CID

5904

Molecular Formula

C₁₆H₁₈N₂O₄S

Molar Mass

334.39 g/mol

State at STP

solid

Appearance

White crystalline powder

Physical Properties

Melting Point

214 °C

Density

1.41 g/cm³

Composition