(Rewrites with revised guidance, paragraphs 1-5)

MEXICO CITY, July 26 (Reuters) - Mexican conglomerate Alfa dropped its guidance for the year on Wednesday due to a challenging commodity market for petrochemicals subsidiary Alpek and following a stark decline in its second-quarter profits.

Alfa lowered its estimated total revenue for the year down to $16.43 billion, from the $17.40 billion it expected at the beginning of the year.

It also lowered its estimated earnings before interest, tax, depreciation and amortization (EBITDA), or core earnings, to $1.53 billion from $1.62 billion.

Chief Executive Alvaro Fernandez said in a statement the drop comes from a slide in expected core earnings from Alpek, "due to a more challenging commodity price environment that has affected volume and margins."

The firm also cut expected capital expenditures for the year to $547 million, trimming projected spending for both Alpek and food subsidiary Sigma.

Nonetheless, Alfa upped projected profits for Sigma on lower raw material costs and the appreciation of the Mexican peso. The peso has strengthened 13% against the U.S. dollar so far this year.

Alfa saw its second-quarter earnings, posted on Wednesday as well, affected by Alpek's weakened performance with the group's second-quarter net profit sliding 96% compared to the year-ago period.

Profit sank to $9 million, a sharp drop from the $243 million net profit recorded in the year-ago quarter. Revenue at the Monterrey-based company fell 10% to $4.22 billion.

Alpek was largely affected by regional raw material cost disparities, which in turn affected prices of polyesther.

Alfa's EBITDA for the quarter fell 46% to $360 million, hit by Alpek's performance but propped up by Sigma, Alfa said.

Both Alpek and Sigma also saw increased extraordinary costs of 921 million pesos and 218 million pesos, respectively, in the quarter, Alfa said in a separate filing.

The conglomerate additionally faced higher financing expenses in the quarter, it said, though income taxes were lower.

($1 = 17.1156 pesos at end-June) (Reporting by Kylie Madry and Aida Pelaez-Fernandez; Editing by Isabel Woodford and Grant McCool)